Fix nonstandard_macro_braces FN on macros with empty args

[profetia]

Closes #15594

changelog: [nonstandard_macro_braces] fix FN on macros with empty args

[rustbot]

r? @Alexendoo

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[profetia]

r? clippy