feat: Support ChatCompletionChunk to LlmResponse conversion

This is part of a larger chain of commits for adding chat completion API support to the Apigee model.

PiperOrigin-RevId: 901419365

Author: google-genai-bot

core/src/main/java/com/google/adk/models/chat/ChatCompletionsCommon.java

@@ -136,7 +136,7 @@ public FunctionCall toFunctionCall(@Nullable String toolCallId) {
       if (name != null) {
         fcBuilder.name(name);
       }
-      if (arguments != null) {
+      if (arguments != null && !arguments.isEmpty()) {
         try {
           Map<String, Object> args =
               objectMapper.readValue(arguments, new TypeReference<Map<String, Object>>() {});