`dcm analyze` false positives

@incendial finding some more false positives here. I can file these somewhere else if that is better for you, just let me know.
```
packages/devtools_app/test/screens/debugger/syntax_highlighter_test.dart (1 issue):
WARNING This test group does not contain any test cases.
at packages/devtools_app/test/screens/debugger/syntax_highlighter_test.dart:514:7
avoid-empty-test-groups : https://dcm.dev/docs/rules/common/avoid-empty-test-groups
```

Start by running `dcm analyze` against `packages/devtools_app/test/screens/debugger/syntax_highlighter_test.dart` and inspect the empty test group reported at line 514. Determine why `avoid-empty-test-groups` flags this case, then verify that analysis no longer reports this false positive while still detecting genuinely empty groups.

索引モデルが issue の本文から書いたものです。