Sync eng/common directory with azure-sdk-tools for PR 8528 (#5755)

* Fix default value for env vars in build-test-resource-config template

* Add empty pool condition

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Co-authored-by: Ben Broderick Phillips <bebroder@microsoft.com>

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

@@ -8,14 +8,15 @@ parameters:
   # EnvVars is used to help diagnose variable conflict issues early
   - name: EnvVars
     type: object
-    default: null
+    default: {}
   - name: SubscriptionConfigurationFilePaths
     type: object
     default: null
 
 steps:
   - task: AzurePowerShell@5
     displayName: Set Pipeline Subnet Info
+    condition: ne(variables['Pool'], '')
     env: ${{ parameters.EnvVars }}
     inputs:
       azureSubscription: azure-sdk-tests