Do not run the matrix if there is no macOS versions picked for benchmarks

[kukushechkin]

After the recent addition of macOS-based benchmarks, the main workflow has been failing with:

Error when evaluating 'strategy' for job 'benchmarks-macos'. apple/swift-nio/.github/workflows/benchmarks.yml@main (Line: 239, Col: 15): Matrix vector 'config' does not contain any values

This happens because swift-nio does not set any _enabled flag for any version of Xcode to run the benchmarks.

An example of a failed run: https://github.com/apple/swift-nio/actions/runs/19479529118

Motivation:

Scheduled jobs should not fail.

Modifications:

Added a condition for running macOS benchmarks matrix generation job.

Result:

Jobs should not fail without any Xcode versions picked for running benchmarks.

An example of a run with macos-matrix job skipped: https://github.com/apple/swift-nio/actions/runs/19482328681/job/55758816398
An example of a run with a opt-in macOS Benchmarks: https://github.com/apple/swift-nio/actions/runs/19484086327/job/55762298924