Sync eng/common directory with azure-sdk-tools for PR 9106 (#1701)

Co-authored-by: Scott Beddall <scbedd@microsoft.com>

eng/common/scripts/Generate-PR-Diff.ps1

@@ -41,8 +41,13 @@ if (!(Test-Path $ArtifactPath))
 $ArtifactPath = Resolve-Path $ArtifactPath
 $ArtifactName = Join-Path $ArtifactPath "diff.json"
 
+$changedFiles = @()
+$changedServices = @()
+
 $changedFiles = Get-ChangedFiles -DiffPath $TargetPath
-$changedServices = Get-ChangedServices -ChangedFiles $changedFiles
+if ($changedFiles) {
+  $changedServices = Get-ChangedServices -ChangedFiles $changedFiles
+}
 
 $result = [PSCustomObject]@{
   "ChangedFiles"    = $changedFiles