[Bug][Go] consider allOf schemas for rendering string default vaules

[jlsherrill]

Fixes #14665 See this issue for reproducer information. This fix does resolve the mentioned issue, but I'm not certain if it is the correct approach.

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[jlsherrill]

FYI technical committee: @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d

[dkliban]

@wing328 Please take a look at this PR.

[wing328]

@jlsherrill thanks for the PR. We can accept this PR and later I will work on another PR to address similar issue/use case in default codegen so that all generators do not need to apply similar change.