fix(protoc-gen-go-http): return nil if err == nil。

[kvii]

Description (what this PR does / why we need it):

Consider these codes that generated by "protoc-gen-go-http":

// "api/helloworld/v1/greeter_http.pb.go"
func (c *GreeterHTTPClientImpl) SayHello(ctx context.Context, in *HelloRequest, opts ...http.CallOption) (*HelloReply, error) {
	var out HelloReply
	pattern := "/helloworld/{name}"
	path := binding.EncodeURL(pattern, in, true)
	opts = append(opts, http.Operation(OperationGreeterSayHello))
	opts = append(opts, http.PathTemplate(pattern))
	err := c.cc.Invoke(ctx, "GET", path, nil, &out, opts...)
	if err != nil {
		return nil, err
	}
	return &out, err // <--
}

It's better to return &out, nil instead of &out, err.

Which issue(s) this PR fixes (resolves / be part of):

Other special notes for the reviewers:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7fa51d) 84.62% compared to head (770425b) 84.72%.

❗ Current head 770425b differs from pull request most recent head dea9814. Consider uploading reports for the commit dea9814 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3188      +/-   ##
==========================================
+ Coverage   84.62%   84.72%   +0.10%     
==========================================
  Files          88       88              
  Lines        3993     3993              
==========================================
+ Hits         3379     3383       +4     
+ Misses        440      438       -2     
+ Partials      174      172       -2     

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