Update eng-common-workflow-enforcer (#14301)

Co-authored-by: Chidozie Ononiwu <chononiw@microsoft.com>

eng/common/pipelines/templates/steps/eng-common-workflow-enforcer.yml

@@ -3,7 +3,7 @@ steps:
     - pwsh: |
         $filesInCommonDir = git diff "origin/$(System.PullRequest.TargetBranch)" HEAD --name-only -- 'eng/common/*'
 
-        if ((-not $PrSourceBranch.StartsWith("sync-eng/common")) -and ($filesInCommonDir.Count -gt 0))
+        if ((!"$(System.PullRequest.SourceBranch)".StartsWith("sync-eng/common")) -and ($LASTEXITCODE -eq 0) -and ($filesInCommonDir.Count -gt 0))
         {
           Write-Host "##vso[task.LogIssue type=error;]Changes to files under 'eng/common' directory should not be made in this Repo`n${filesInCommonDir}"
           Write-Host "##vso[task.LogIssue type=error;]Please follow workflow at https://github.com/Azure/azure-sdk-tools/blob/master/doc/common/common_engsys.md"