fix: single option in #drilldownlink produces error

* Update ProcessTemplate.php

Fixing fatal error while going to Special:BrowseData/Thing?_single since $vm is null (and null values can't be concated to an array)

includes/Specials/BrowseData/ProcessTemplate.php

@@ -35,7 +35,7 @@ public function __invoke( $template, $vm ) {
 			$msg[ "msg_$message" ] = wfMessage( $message )->text();
 		}
 
-		$html = $templateParser->processTemplate( $template, $vm + $msg );
+		$html = $templateParser->processTemplate( $template, (array)$vm + $msg );
 
 		// Remove leading whitespace from all lines as the parser will interpret lines containing
 		// text starting with whitespace as preformatted text later;