Correctly encode structs without members

The JSON RPC API specification for 'eth_signTypedData_v4' does not
explicitly state that the required struct 'EIP712Domain' has to define
any members [1]. The current implementation cannot correctly encode
such a structure, as it assumes that at least one member has been
parsed and added to the struct's string representation.

This commit refactors the struct encoding function such that it now
accepts structures that does not define any members.

[1] https://eips.ethereum.org/EIPS/eip-712#specification-of-the-eth_signtypeddata-json-rpc

crypto/src/main/java/org/web3j/crypto/StructuredDataEncoder.java

@@ -24,6 +24,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.StringJoiner;
 import java.util.TreeSet;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -120,14 +121,10 @@ public Set<String> getDependencies(String primaryType) {
     public String encodeStruct(String structName) {
         HashMap<String, List<StructuredData.Entry>> types = jsonMessageObject.getTypes();
 
-        StringBuilder structRepresentation = new StringBuilder(structName + "(");
+        StringJoiner structRepresentation = new StringJoiner(",", structName + "(", ")");
         for (StructuredData.Entry entry : types.get(structName)) {
-            structRepresentation.append(String.format("%s %s,", entry.getType(), entry.getName()));
+            structRepresentation.add(String.format("%s %s", entry.getType(), entry.getName()));
         }
-        structRepresentation =
-                new StringBuilder(
-                        structRepresentation.substring(0, structRepresentation.length() - 1));
-        structRepresentation.append(")");
 
         return structRepresentation.toString();
     }

crypto/src/test/java/org/web3j/crypto/StructuredDataTest.java

@@ -113,6 +113,20 @@ public void testEncodeType() throws IOException, RuntimeException {
                 expectedTypeEncoding);
     }
 
+    @Test
+    public void testMinimalEncodeType() throws IOException, RuntimeException {
+        String validMinimalStructuredDataJSONFilePath =
+                "build/resources/test/"
+                        + "structured_data_json_files/ValidMinimalStructuredData.json";
+        StructuredDataEncoder dataEncoder =
+                new StructuredDataEncoder(getResource(validMinimalStructuredDataJSONFilePath));
+        String expectedTypeEncoding = "EIP712Domain()";
+
+        assertEquals(
+                dataEncoder.encodeType(dataEncoder.jsonMessageObject.getPrimaryType()),
+                expectedTypeEncoding);
+    }
+
     @Test
     public void testTypeHash() throws IOException, RuntimeException {
         StructuredDataEncoder dataEncoder = new StructuredDataEncoder(jsonMessageString);

crypto/src/test/resources/structured_data_json_files/ValidMinimalStructuredData.json

@@ -0,0 +1,8 @@
+{
+  "types": {
+    "EIP712Domain": []
+  },
+  "primaryType": "EIP712Domain",
+  "domain": {},
+  "message": {}
+}