Handle nil requests in GRPC client method builder

[delkopiso]

What's up

The GRPC client remote method builder function casts the request interface into a pointer to the proto request struct. However, if the endpoint's input is an empty message then the client will be passing in a nil request interface. This causes a panic at the site of the interface cast.

What this does

Checks if the proto request obj is nil before casting. If it's nil, then we return a pointer to the empty proto request struct instead of casting the request interface to it.

Before

// BuildDoStuffFunc builds the remote method to invoke for "dummy" service
// "doStuff" endpoint.
func BuildDoStuffFunc(grpccli dummypb.DummyClient, cliopts ...grpc.CallOption) goagrpc.RemoteFunc {
	return func(ctx context.Context, reqpb interface{}, opts ...grpc.CallOption) (interface{}, error) {
		for _, opt := range cliopts {
			opts = append(opts, opt)
		}
		return grpccli.DoStuff(ctx, reqpb.(*dummypb.DoStuffRequest), opts...)
	}
}

After

// BuildDoStuffFunc builds the remote method to invoke for "dummy" service
// "doStuff" endpoint.
func BuildDoStuffFunc(grpccli dummypb.DummyClient, cliopts ...grpc.CallOption) goagrpc.RemoteFunc {
	return func(ctx context.Context, reqpb interface{}, opts ...grpc.CallOption) (interface{}, error) {
		for _, opt := range cliopts {
			opts = append(opts, opt)
		}
		if reqpb != nil {
			return grpccli.DoStuff(ctx, reqpb.(*dummypb.DoStuffRequest), opts...)
		}
		return grpccli.DoStuff(ctx, &dummypb.DoStuffRequest{}, opts...)
	}
}

Fixes #2185

[raphael]

This is great, thank you! Do you think you could back-port this fix to v2 as well? (simply create a branch off of v2 and cherry-pick this merge commit).

[delkopiso]

@raphael yeah, just created #2187