lib/types/pluginLuaConfig: only merge pre and post when defined

[MattSturgeon]

Follow up to #1876 (comment)

• Have pre and post default to null
• Only merge them into content when non-null
• Mention this in content's description

cc @traxys

[MattSturgeon]

@Mergifyio queue

[mergify]

queue

✅ The pull request has been merged automatically

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The pull request has been merged automatically at 76df096

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[stasjok]

Since they are in mkMerge, maybe it's faster to use lib.optional?

[MattSturgeon]

I generally default to using mkIf to allow the module system to "do its thing". I guess in this case there's nothing really for the module system to do, so using optional(s) should result in less work done.

Not sure if there's a good way to measure such performance differences though.