fix: Avoid NPE for gRPC when acceptedOutputModes isn't specified for the client side (#256)

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Author: fjuma

spec-grpc/src/main/java/io/a2a/grpc/utils/ProtoUtils.java

@@ -337,7 +337,9 @@ private static io.a2a.grpc.AuthenticationInfo authenticationInfo(PushNotificatio
 
         public static io.a2a.grpc.SendMessageConfiguration messageSendConfiguration(MessageSendConfiguration messageSendConfiguration) {
             io.a2a.grpc.SendMessageConfiguration.Builder builder = io.a2a.grpc.SendMessageConfiguration.newBuilder();
-            builder.addAllAcceptedOutputModes(messageSendConfiguration.acceptedOutputModes());
+            if (messageSendConfiguration.acceptedOutputModes() != null) {
+                builder.addAllAcceptedOutputModes(messageSendConfiguration.acceptedOutputModes());
+            }
             if (messageSendConfiguration.historyLength() != null) {
                 builder.setHistoryLength(messageSendConfiguration.historyLength());
             }
@@ -772,7 +774,8 @@ private static AgentExtension agentExtension(io.a2a.grpc.AgentExtension agentExt
 
         private static MessageSendConfiguration messageSendConfiguration(io.a2a.grpc.SendMessageConfiguration sendMessageConfiguration) {
             return new MessageSendConfiguration(
-                    new ArrayList<>(sendMessageConfiguration.getAcceptedOutputModesList()),
+                    sendMessageConfiguration.getAcceptedOutputModesList().isEmpty() ? null :
+                            new ArrayList<>(sendMessageConfiguration.getAcceptedOutputModesList()),
                     sendMessageConfiguration.getHistoryLength(),
                     pushNotification(sendMessageConfiguration.getPushNotification()),
                     sendMessageConfiguration.getBlocking()

tests/server-common/src/test/java/io/a2a/server/apps/common/AbstractA2AServerTest.java

@@ -1314,7 +1314,6 @@ private AgentCard createTestAgentCard() {
     private ClientConfig createClientConfig(boolean streaming) {
         return new ClientConfig.Builder()
                 .setStreaming(streaming)
-                .setAcceptedOutputModes(List.of("text"))
                 .build();
     }