fix: Action breaks when flutter_version is not informed

[tyemy]

Description In the documentation flutter_version is optional but when is not informed Setup Flutter step fails.

Steps To Reproduce

1. Add a workflow using flutter_package.yml
2. Do not inform flutter_version
3. See error

Example:

jobs:
  build:
    uses: VeryGoodOpenSource/very_good_workflows/.github/workflows/flutter_package.yml@v1
    with:
      coverage_excludes: "*.g.dart"
      test_recursion: true

Screenshots If applicable, add screenshots to help explain your problem.

[wolfenrain]

Seeing the same sort of problem but slightly different. If I don't specify the flutter version the Setup Flutter stage does succeed but the next stage fails:

[wolfenrain]

I am not able to reproduce this anymore, it seems to have been solved in the internal workflow that is being used to get the Flutter version.

Closing this issue, if you are observing something similiar to this please open a new issue so we can investigate it then <3