Add lib.fileset.optional :: Bool -> FileSet -> FileSet

[infinisil]

OT: I would quite like to have fileset.optional : Bool -> FileSet -> FileSet such that optional false x == fileset.empty. Example:

mkDerivation(finalAttrs: {
  src = fileset.unions [
    ./src
    (fileset.optional finalAttrs.doCheck ./tests)
  ];
})

Originally posted by @roberth in #265964 (comment)

[infinisil]

OT: I would quite like to have fileset.optional : Bool -> FileSet -> FileSet such that optional false x == fileset.empty

Is this actually required? You can already use lib.optional(s) and have the result coerced to a fileset.

Originally posted by @alyssais in #265964 (comment)

[infinisil]

@alyssais A wrapping lib.unions is needed to convert the list returned by lib.optional[s] to a file set. Though I have also played around with the thought of implicitly coercing lists of filesets to their union, which would remove the need for that.

[alyssais]

Though I have also played around with the thought of implicitly coercing lists of filesets to their union, which would remove the need for that.

I'd like that. I didn't even realise that individual paths are coerced, but lists aren't — that's counterintuitive to me.