Incomplete `macro_rules` definition is deleted

[Jarcho]

Given a macro_rules definition containing only a matcher without a body, rustfmt will delete the matcher text. e.g.

macro_rules! foo { (foo) }

Is converted to:

macro_rules! foo {}

This happens for any brace style, but only if they're the last token. e.g. the following works properly:

macro_rules! foo { (foo) => }

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using rustfmt 1.7.0-stable (82e1608d 2023-12-21)

[ytmimi]

seems related to changes made in #5883. probably need to go back and update the logic to take incomplete matchers into account.