[3.10] [PHP 8.1] Fixes com_finder/indexer/query (joomla#36796)

* [3.10] [PHP 8.1] Fixes com_finder/indexer/query

Fixes `Deprecated: html_entity_decode(): Passing null to parameter #1 ($string) of type string is deprecated in administrator/components/com_finder/helpers/indexer/query.php on line 733`

* Better fix for joomla#36796 as suggested by @zero-24

administrator/components/com_finder/helpers/indexer/query.php

@@ -172,7 +172,7 @@ class FinderIndexerQuery
 	public function __construct($options)
 	{
 		// Get the input string.
-		$this->input = isset($options['input']) ? $options['input'] : null;
+		$this->input = isset($options['input']) ? $options['input'] : '';
 
 		// Get the empty query setting.
 		$this->empty = isset($options['empty']) ? (bool) $options['empty'] : false;