When gRPC backend is not reachable, a gRPC call may gets back before any trailer gets initialized. Adds a nullptr check to avoid segfault.
When gRPC backend is not reachable, a gRPC call may gets back before any
trailer gets initialized. Adds a nullptr check to avoid segfault.

grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web,grpc/grpc-web