Do not run the matrix if there is no macOS versions picked for benchmarks (#3450)

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

Author: kukushechkin

.github/workflows/benchmarks.yml

@@ -149,6 +149,12 @@ jobs:
   construct-matrix-macos:
     name: Construct macOS Benchmarks matrix
     runs-on: ubuntu-latest
+    if: |
+      inputs.macos_xcode_16_3_enabled ||
+      inputs.macos_xcode_16_4_enabled ||
+      inputs.macos_xcode_26_0_enabled ||
+      inputs.macos_xcode_26_1_enabled ||
+      inputs.macos_xcode_latest_beta_enabled
     outputs:
       macos-matrix: '${{ steps.generate-matrix.outputs.macos-matrix }}'
     steps: