fix: nil pointer dereferencing in writeArg

[aliforever]

• Added example for hset using struct
• Added nil pointer checks to WriteArg (internal/proto/writer.go:62)
• Added test cases

Issue: #3208

[ndyakov]

Hello @aliforever, Looking at the example test, don't you think we should add a check with omitempty to verify the nil pointers will be retrieved as such? Other than that, lgtm. Maybe remove the dump package from the test so the linter passes? For some reason it doesn't like it. Will review once we have that test updated.