Baseline PR Tool should close empty PRs

[ellahathaway]

In dotnet/dotnet#7628 (a baseline update PR), the first commit modified the baseline, and the second commit reverted those changes, resulting in a PR with no effective changes. The baseline PR tool should have detected that there were no differences from the target branch, closed the PR, and posted a comment explaining why.

[mthalman]

This was a consequence of the incorrect behavior identified by dotnet/dotnet#7632. That said, there are possible legitimate scenarios where an update causes no diff to occur.