Release pipeline validation run fails PR creation when no new changes

[mthalman]

The SB release pipeline has a mode that can be configured for PR validation. In the "Release" stage of the pipeline, it creates the Source Build PR to update the sdk repo to use the released SDK version (e.g. global.json update). When doing so, it uses a hardcoded release version to test with (example (internal link)). If that version, for example 9.0.100-rc.1, matches the version that already exists in the target release branch, then there will be no commits differences between the release branch and the PR branch.

This results in a failure in the step to create the PR:

pull request create failed: GraphQL: No commits between release/9.0.1xx and oct2024-source-build-1729871686 (createPullRequest)

Example build

[ellahathaway]

Rather than failing, maybe we should warn when this happens? I don't think that we should let the step be green if no PR is created, but I also don't think that a failure is correct either.

[mthalman]

PR to fix this: https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/49219