validate-pull-request adds "Fixes #0" to the PR description if no issue is found (OSOE-745)

[Piedone]

The validate-pull-request workflow, specifically Update-GitHubIssueAndPullRequest adds here a "Fixes #0" message to the PR's description if no matching issue is found. This might happen only if there are no issues at all (but the issues feature is enabled) because e.g. here this didn't happen.

Jira issue

[BenedekFarkas]

That's weird, I don't have a local repro in either PS or pwsh and in the "no issues at all" vs. "there are issues, but none match" cases, so the check on line 55 worked.

[Piedone]

FYI under the internal issue I added some examples of this happening in a private repo.