[go][model] Use pointers to avoid circular references

[wy-z]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

• Use pointers to avoid circular references
• Abandon omitempty tag when the field is required

[wy-z]

@wing328 It seems that #4440 has not modified model.mustache, so this PR may be useful~

[wy-z]

@wing328 After reading #2330 carefully, I found that the problem was more complex than I thought~
I closed the PR.