handle empty arguments in switch statements

[markromanmiller]

I've noticed that there was a conflict between the comma spacing linter and the infix operator linter when using switch statements that have fall-through arguments like switch(foo, x = , y = bar, z = baz). If there is a space in the x argument between the = and the ,, then the comma spacing linter complains; if there is no space there the infix operator linter complains.

I consulted the vctrs documentation here (because that's what originally made me use switch) and saw it was written with the space, so I modified the comma linter to allow these types of lines.

I'm happy to take feedback on my approach; I'm not too confident using base R.

[jimhester]

Looks good to me, can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[markromanmiller]

Changes made! Thanks.

[jimhester]

Thanks for working on this!

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