Fixes readwrite-splitting data sources is incorrect when tableless

[JoshuaChen]

Fix #34387

When using the following configuration, the value passed to rule.findDataSourceGroupRule(logicDataSourceName) is write_ds or read_ds_0, which fails to locate the correct dataSourceGroups (e.g., joshua in the configuration). This causes the ReadwriteSplitting logic to be skipped. This PR fixes the issue by ensuring the logicDataSourceName parameter passed is set to joshua.

mode:
  type: Standalone
  repository:
    type: JDBC

dataSources:
  write_ds:
    dataSourceClassName: com.zaxxer.hikari.HikariDataSource
    url: jdbc:postgresql://127.0.0.1:5432/mydatabase
    username: myuser
    password: secret
  read_ds_0:
    dataSourceClassName: com.zaxxer.hikari.HikariDataSource
    url: jdbc:postgresql://127.0.0.1:5432/mydatabase
    username: myuser
    password: secret
    readOnly: true

rules:
  - !READWRITE_SPLITTING
    dataSourceGroups:
      "joshua":
        writeDataSourceName: write_ds
        readDataSourceNames:
          - read_ds_0
        transactionalReadQueryStrategy: PRIMARY
        loadBalancerName: baseAA
    loadBalancers:
      baseAA: # Load balance algorithm name
        type: ROUND_ROBIN
  - !SINGLE
    tables:
      - "*.*.*"

props:
  sql-show: true

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