[DevOp] Allow $amarinStorage to be empty. (#8963)

pwsh will complain if a mandatory parameter is empty. Remove the attr
and allow it to be empty since the if statements do check fir it.

tools/devops/device-tests/scripts/GitHub.psm1

@@ -306,13 +306,12 @@ function New-GitHubSummaryComment {
         [String]
         $Context,
 
-        [Parameter(Mandatory)]
-        [String]
-        $XamarinStoragePath,
-
         [Parameter(Mandatory)]
         [String]
         $TestSummaryPath
+
+        [String]
+        $XamarinStoragePath,
     )
 
     $envVars = @{