GrpcExceptionHandledServerCall tries to merge null trailers

[emil-e]

At...

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

Line 39 in b3eb9c2

trailers.merge(statusException.getTrailers());

statusException.getTrailers() can (and very often is) null, thus the @Nullable annotation. This causes exceptions since Metadata.merge does not deal with null values:

Exception in thread "DefaultDispatcher-worker-1" java.lang.NullPointerException: Cannot invoke "io.grpc.Metadata.isEmpty()" because "other" is null
at io.grpc.Metadata.merge(Metadata.java:508)
at org.springframework.grpc.server.exception.GrpcExceptionHandledServerCall.close(GrpcExceptionHandledServerCall.java:39)
at io.grpc.PartialForwardingServerCall.close(PartialForwardingServerCall.java:48)
at io.grpc.ForwardingServerCall.close(ForwardingServerCall.java:22)
at io.grpc.ForwardingServerCall$SimpleForwardingServerCall.close(ForwardingServerCall.java:44)
at io.micrometer.core.instrument.binder.grpc.ObservationGrpcServerCall.close(ObservationGrpcServerCall.java:71)
at io.grpc.kotlin.ServerCalls$serverCallListener$rpcJob$1.invokeSuspend(ServerCalls.kt:268)

[therepanic]

Hi, @emil-e. I believe that if statusException.getTrailers() == null, then we shouldn't merge it? I attached PR, you can take a look.

[emil-e]

That's exactly my take, yes. LGTM.

[onobc]

This is an excellent case where JSpecify would have caught this (#192 coming soon).