Fix default value for env vars in build-test-resource-config template (#8528)

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

* Add empty pool condition

* Write delete failures as warnings

Author: benbp

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

eng/scripts/live-test-resource-cleanup.ps1

@@ -441,7 +441,8 @@ function DeleteAndPurgeGroups([array]$toDelete) {
         }
       }
     } catch {
-      Write-Error $_
+      Write-Warning "Failure deleting/purging group $($rg.ResourceGroupName):"
+      Write-Warning $_
       $hasError = $true
     }
   }