mkNeovimPlugin: refactor lua code generation logic

[GaetanLepage]

Follow-up of #2619

This keeps the current behavior of not adding a plugins.*.luaConfig option if it doesn't make sense (for rustacenvim for e.g.).
What has been removed is the ability/expectation for configLocation to be null.

[MattSturgeon]

Q: why is it ok to remove the null check? (configLocation != null)

[GaetanLepage]

Because it should never be null.
null is a forbidden value. Or at least, it is fine as long as hasLuaConfig is false.

[traxys]

I made #2624 to see how we could implement a luaConfig attribute for plugins that are configured with globals. If the design is good enough maybe we should have an argument of mkNeovimPlugin which would allow rustaceanvim to use that?

[GaetanLepage]

@Mergifyio queue

[mergify]

queue

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The pull request has been merged automatically at cf7e026

[mergify]

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