Fix error when executing method SSViewer::templates() when $subTemplates is still null

Without the fix this causes the framework to throw a 500 error:
`ERROR [Warning]: array_merge(): Expected parameter 2 to be an array, null given`

I noticed the error while doing a search with the `GridFieldAddExistingAutocompleter`. The error probably gets supressed on live environments.

src/View/SSViewer.php

@@ -167,7 +167,7 @@ class SSViewer implements Flushable
      *
      * @var array
      */
-    protected $subTemplates = null;
+    protected $subTemplates = [];
 
     /**
      * @var bool