[BugFix] Fix exception when loading empty json string to json column

[banmoy]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

When loading empty json string to json column, there will be the following exception.

java.lang.StringIndexOutOfBoundsException: String index out of range: 0
	at java.lang.String.charAt(String.java:658)
	at com.starrocks.connector.flink.row.sink.StarRocksTableRowTransformer.typeConvertion(StarRocksTableRowTransformer.java:135)
	at com.starrocks.connector.flink.row.sink.StarRocksTableRowTransformer.transform(StarRocksTableRowTransformer.java:96)
	at com.starrocks.connector.flink.row.sink.StarRocksTableRowTransformer.transform(StarRocksTableRowTransformer.java:50)
	at com.starrocks.connector.flink.table.sink.StarRocksDynamicSinkFunctionV2.invoke(StarRocksDynamicSinkFunctionV2.java:164)
	at org.apache.flink.table.runtime.operators.sink.SinkOperator.processElement(SinkOperator.java:65)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.pushToOperator(CopyingChainingOutput.java:82)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:57)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:29)
	at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:56)
	at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:29)
	at org.apache.flink.table.runtime.util.StreamRecordCollector.collect(StreamRecordCollector.java:44)
	at org.apache.flink.table.runtime.operators.sink.ConstraintEnforcer.processElement(ConstraintEnforcer.java:247)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.pushToOperator(CopyingChainingOutput.java:82)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:57)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:29)
	at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:56)
	at org.apache.flink.streaming.api.operators.CountingOutput.collect(CountingOutput.java:29)
	at StreamExecCalc$50.processElement_split4(Unknown Source)
	at StreamExecCalc$50.processElement(Unknown Source)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.pushToOperator(CopyingChainingOutput.java:82)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:57)
	at org.apache.flink.streaming.runtime.tasks.CopyingChainingOutput.collect(CopyingChainingOutput.java:29)
	at org.apache.flink.streaming.runtime.tasks.SourceOperatorStreamTask$AsyncDataOutputToOutput.emitRecord(SourceOperatorStreamTask.java:313)
	at org.apache.flink.streaming.api.operators.source.SourceOutputWithWatermarks.collect(SourceOutputWithWatermarks.java:110)
	at org.apache.flink.streaming.api.operators.source.SourceOutputWithWatermarks.collect(SourceOutputWithWatermarks.java:101)
	at org.apache.flink.cdc.connectors.mysql.source.reader.MySqlRecordEmitter$OutputCollector.collect(MySqlRecordEmitter.java:141)
	at org.apache.flink.cdc.debezium.table.AppendMetadataCollector.collect(AppendMetadataCollector.java:59)
	at org.apache.flink.cdc.debezium.table.RowDataDebeziumDeserializeSchema.emit(RowDataDebeziumDeserializeSchema.java:170)
	at org.apache.flink.cdc.debezium.table.RowDataDebeziumDeserializeSchema.deserialize(RowDataDebeziumDeserializeSchema.java:129)
	at org.apache.flink.cdc.connectors.mysql.source.reader.MySqlRecordEmitter.emitElement(MySqlRecordEmitter.java:119)
	at org.apache.flink.cdc.connectors.mysql.source.reader.MySqlRecordEmitter.processElement(MySqlRecordEmitter.java:101)
	at org.apache.flink.cdc.connectors.mysql.source.reader.MySqlRecordEmitter.emitRecord(MySqlRecordEmitter.java:73)
	at org.apache.flink.cdc.connectors.mysql.source.reader.MySqlRecordEmitter.emitRecord(MySqlRecordEmitter.java:46)
	at org.apache.flink.connector.base.source.reader.SourceReaderBase.pollNext(SourceReaderBase.java:143)
	at org.apache.flink.streaming.api.operators.SourceOperator.emitNext(SourceOperator.java:385)
	at org.apache.flink.streaming.runtime.io.StreamTaskSourceInput.emitNext(StreamTaskSourceInput.java:68)
	at org.apache.flink.streaming.runtime.io.StreamOneInputProcessor.processInput(StreamOneInputProcessor.java:65)
	at org.apache.flink.streaming.runtime.tasks.StreamTask.processInput(StreamTask.java:542)
	at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor.runMailboxLoop(MailboxProcessor.java:231)
	at org.apache.flink.streaming.runtime.tasks.StreamTask.runMailboxLoop(StreamTask.java:831)
	at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:780)
	at org.apache.flink.runtime.taskmanager.Task.runWithSystemExitMonitoring(Task.java:935)
	at org.apache.flink.runtime.taskmanager.Task.restoreAndInvoke(Task.java:914)
	at org.apache.flink.runtime.taskmanager.Task.doRun(Task.java:728)
	at org.apache.flink.runtime.taskmanager.Task.run(Task.java:550)
	at java.lang.Thread.run(Thread.java:748)

Problem Summary(Required) ：

need to check string length before accessing its elements

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function