LintDiff check should fail if it ran no validations

We want to do it as a form of "last-resort", "catch-call" protection while we work through LintDiff reliability issues:

- https://github.com/Azure/azure-sdk-tools/issues/7731

Of course, this means that if LintDiff correctly did not validate anything, because e.g. a PR doesn't touch any specs, this will result in false positive that will have to be suppressed with a label per https://aka.ms/pr-suppressions. We can deal with this for now. Perhaps we could make the check a bit smarter, by looking if the PR changes any files in `specification` directory. Relevant code and recent work to achieve that:

- [`const changedFiles: DiffFileResult | undefined = await this.localPRContext?.getChangingFiles();`](https://devdiv.visualstudio.com/DevDiv/_git/openapi-alps?path=/public/swagger-validation-common/src/context.ts&version=GC3a49982c0758967e3fc464ab0e0aba73d1883af3&line=464&lineEnd=465&lineStartColumn=1&lineEndColumn=1&lineStyle=plain&_a=contents)
- [Pull Request 531898](https://devdiv.visualstudio.com/DevDiv/_git/openapi-alps/pullrequest/531898): Fix bug where changes to unmodified README files were not picked up by LintDiff.

This issue is very related to:
- https://github.com/Azure/azure-sdk-tools/issues/7761

Note that even if we implement this issue, we may still have gaps. E.g. if a PR edits existing specs and adds new specs that haven't been added to README, LintDiff will run and we will still have a gap. That's why the issue #7761 also needs to be implemented for full coverage.

Note that LintDiff may detect existing specs if the source/base branch of given PR is behind the target branch, [as in this example](https://github.com/Elina-L/azure-rest-api-specs/tree/wenxliu/SpotPlacementRecommenderAPI) for [PR 27899](https://github.com/Azure/azure-rest-api-specs/pull/27899):


Hence this gap may be in fact more prevalent than it seems at first.