Sync eng/common directory with azure-sdk-tools for PR 8516 (#5752)

* Ensure subConfigFiles is not an empty string

* Skip instances where $file is an empty string

---------

Co-authored-by: Daniel Jurek <djurek@microsoft.com>

eng/common/TestResources/build-test-resource-config.yml

@@ -68,6 +68,12 @@ steps:
       if ($subConfigFilesRaw) {
         $subConfigFiles = $subConfigFilesRaw | ConvertFrom-Json -AsHashtable
         foreach ($file in $subConfigFiles) {
+          # In some cases, $file could be an empty string. Get-Content will fail
+          # if $file is an empty string, so skip those cases.
+          if (!$file) {
+            continue
+          }
+
           Write-Host "Merging sub config from file: $file"
           $subConfig = Get-Content $file | ConvertFrom-Json -AsHashtable
           $finalConfig = UpdateSubscriptionConfiguration $finalConfig $subConfig