Fix addition of assigned roles list (Azure#14924)

Fixing addition of the lists. List could return a single item and added @() to handle such cases

src/StackHCI/Az.StackHCI.psm1

@@ -478,7 +478,7 @@ param(
     $appSP = Retry-Command -ScriptBlock { Get-AzureADServicePrincipal -Filter "AppId eq '$AppId'"}
 
     # Try Get-AzureADServiceAppRoleAssignment as well to get app role assignments. WAC token falls under this case.
-    $assignedPerms = Retry-Command -ScriptBlock { (Get-AzureADServiceAppRoleAssignedTo -ObjectId $appSP.ObjectId) + (Get-AzureADServiceAppRoleAssignment -ObjectId $appSP.ObjectId)} -RetryIfNullOutput $false
+    $assignedPerms = Retry-Command -ScriptBlock { @(Get-AzureADServiceAppRoleAssignedTo -ObjectId $appSP.ObjectId) + @(Get-AzureADServiceAppRoleAssignment -ObjectId $appSP.ObjectId)} -RetryIfNullOutput $false
 
     $clusterRead = $assignedPerms | where { ($_.Id -eq $ClusterReadPermission) }
     $clusterReadWrite = $assignedPerms | where { ($_.Id -eq $ClusterReadWritePermission) }