Avoid NPE when merging trailers in GrpcExceptionHandledServerCall

This commit adds a null check on the trailers returns by the status exception in
`GrpcExceptionHandledServerCall` and only merges the exceptions trailers
when they are non-null.

Fixes #229

Signed-off-by: Andrey Litvitski <[email protected]>

Author: therepanic

spring-grpc-core/src/main/java/org/springframework/grpc/server/exception/GrpcExceptionHandledServerCall.java

@@ -36,7 +36,10 @@ public void close(Status status, Metadata trailers) {
 		if (status.getCode() == Status.Code.UNKNOWN && status.getCause() != null) {
 			final Throwable cause = status.getCause();
 			final StatusException statusException = this.exceptionHandler.handleException(cause);
-			trailers.merge(statusException.getTrailers());
+			Metadata statusExceptionTrailers = statusException.getTrailers();
+			if (statusExceptionTrailers != null) {
+				trailers.merge(statusExceptionTrailers);
+			}
 			super.close(statusException.getStatus(), trailers);
 		}
 		else {