[ARM Incremental TypeSpec] PRs changing *.tsp but no swagger should return "true"

This PR changes `*.tsp` and `tspconfig.yaml`, but no changes to generated swagger:

- https://github.com/Azure/azure-rest-api-specs/pull/39031

Check `ARM Incremental TypeSpec` was intended to include this case, we just didn't have a test for it.

We wanted to be careful to not add auto-signoff for TSP conversions, so we verify any modified swagger was already TSP-generated in the base branch.

This left an edge case for PRs which don't modify any swagger, to be conservative we mapped this to "no auto-signoff", but maybe we just map this to "yes auto-signoff"?

https://github.com/Azure/azure-rest-api-specs/blob/e22dcfdafa3316a9ae96c7fd02318ee87041b409/.github/workflows/src/arm-auto-signoff/arm-incremental-typespec.js#L107-L117