Fix crash if a non-module is passed to TransformModuleFilesToModel

[fsedano]

If a regular schema file, instead of a module, was passed to TransformModuleFilesToModel, it would crash.

Description

The change checks the metadata for the module, if it's null it returns the proper error.

References

Fixes #384

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[linux-foundation-easycla]

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[molssongroup]

/easycla