fix: avoid setting Part.partMetadata when metadata is empty

Author: hemasekhar-p

a2a/src/main/java/com/google/adk/a2a/converters/PartConverter.java

@@ -87,10 +87,12 @@ public static com.google.genai.types.Part toGenaiPart(io.a2a.spec.Part<?> a2aPar
       com.google.genai.types.Part.Builder partBuilder =
           com.google.genai.types.Part.builder().text(textPart.getText());
       if (textPart.getMetadata() != null) {
-        partBuilder.partMetadata(textPart.getMetadata());
         if (Objects.equals(textPart.getMetadata().get("thought"), true)) {
           partBuilder.thought(true);
         }
+        if (!textPart.getMetadata().isEmpty()) {
+          partBuilder.partMetadata(textPart.getMetadata());
+        }
       }
       return partBuilder.build();
     }
@@ -122,7 +124,7 @@ private static com.google.genai.types.Part convertFilePartToGenAiPart(FilePart f
                       .fileUri(fileWithUri.uri())
                       .mimeType(fileWithUri.mimeType())
                       .build());
-      if (metadata != null) {
+      if (metadata != null && !metadata.isEmpty()) {
         builder.partMetadata(metadata);
       }
       return builder.build();
@@ -137,7 +139,7 @@ private static com.google.genai.types.Part convertFilePartToGenAiPart(FilePart f
       com.google.genai.types.Part.Builder builder =
           com.google.genai.types.Part.builder()
               .inlineData(Blob.builder().data(decoded).mimeType(fileWithBytes.mimeType()).build());
-      if (metadata != null) {
+      if (metadata != null && !metadata.isEmpty()) {
         builder.partMetadata(metadata);
       }
       return builder.build();

a2a/src/test/java/com/google/adk/a2a/converters/PartConverterTest.java

@@ -483,6 +483,27 @@ public void toGenaiPart_dataPartWithNonMapCoercedToMap() {
     assertThat(result.functionCall().get().args()).hasValue(ImmutableMap.of("value", 123));
   }
 
+  @Test
+  public void toGenaiPart_withTextPartEmptyMetadata_doesNotSetPartMetadata() {
+    TextPart textPart = new TextPart("Hello", ImmutableMap.of());
+
+    Part result = PartConverter.toGenaiPart(textPart);
+
+    assertThat(result.text()).hasValue("Hello");
+    assertThat(result.partMetadata()).isEmpty();
+  }
+
+  @Test
+  public void toGenaiPart_withFilePartEmptyMetadata_doesNotSetPartMetadata() {
+    FilePart filePart =
+        new FilePart(
+            new FileWithUri("text/plain", "file.txt", "http://file.txt"), ImmutableMap.of());
+
+    Part result = PartConverter.toGenaiPart(filePart);
+
+    assertThat(result.partMetadata()).isEmpty();
+  }
+
   @Test
   public void toGenaiPart_withTextPartMetadata_propagatesMetadata() {
     TextPart textPart = new TextPart("Hello", ImmutableMap.of("key", "value"));