gen: Avoid pointer to slices for nullable unions.

[0xjac]

Related to #371 and #372, when generating a struct with a field of type ["null", "array<string>"] it would be great if there was an option to have the resulting go type be []string instead of *[]string since Go slices can be nil directly.

[0xjac]

@nrwiersma If you are interested, I'm happy to submit a PR with this (with tests), including hiding the new behavior behind a feature flag (like -plain-slices) to preserve the existing behavior by default.

[nrwiersma]

Sure, that would be great. I have unfortunately not had time to look at these at the moment.