Fix bug in StaticClosure sniff

[tfrommen]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This PR fixes a bug with a non-static fn arrow function using implicit return, fn () => 'foo', that could be static.

What is the current behavior? (You can also link to an open issue here)

Given this code:

fn () => 'foo'

phpcs will correctly report the closure being non-static, while it could. However, running phpcbf will result in the following invalid code:

static function () => 'foo'

Notice the implicit return that is invalid for a proper function.

What is the new behavior (if this is a feature change)?

With the PR merged, the fixed code will look like so:

static fn () => 'foo'

The closure is still an arrow function, making use of implicit return, but now static.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

Other information: