Fix bug in handling repos-file input

[alejandrohdezma]

🐛 The bug

Inputs read by core.getInput are initialized as '' if they aren't present. However we are handling them as undefined:

const file: string | undefined = core.getInput('repos-file')

✔️ The solution

First of all action.yml shouldn't contain any default: '' for non-required inputs, since that is already the initial value.

Second of all inputs should be read as string and checked using:

const file: string = core.getInput('repos-file')

if (!file) {
  //Input is not provided
}

What other changes are included in this PR?

A whole suite of tests for the check.repoFiles() method has been included so we ensure this error won't happen again in the future.

References

This fixes #21