Fix no quotas empty array error

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

lib/Service/MailManager.php

@@ -414,6 +414,11 @@ class MailManager implements IMailManager {
 			];
 		}, array_merge(...array_values($quotas)));
 
+		if (empty($storageQuotas)) {
+			// Nothing left to do, and array_merge doesn't like to be called with zero arguments.
+			return null;
+		}
+
 		/**
 		 * Deduplicate identical quota roots
 		 */