Harmonize default value for also_notify

The template code that processes also_notify is of a kind with the checks and
processing of other optional smart-array values. Make its default an empty
string so that the clause may be properly omitted from a config that doesn't
use it.

manifests/view.pp

@@ -10,7 +10,7 @@ define bind::view (
     $recursion_match_destinations = '',
     $recursion_match_only         = false,
     $notify_source                = '',
-    $also_notify                  = [],
+    $also_notify                  = '',
     $empty_zones                  = '',
     $order                        = '10',
     $minimal_responses            = false,