Unset $retval after HOOK_FORMAT_ENCLOSURES.

Fixes an edge case where changing the array of enclosures with HOOK_FORMAT_ENCLOSURES might break the dropdown rendering if HOOK_RENDER_ENCLOSURE has no registered hooks.

include/functions2.php

@@ -1890,6 +1890,7 @@
 				$rv = $retval;
 			}
 		}
+		unset($retval); // Unset to prevent breaking render if there are no HOOK_RENDER_ENCLOSURE hooks below.
 
 		if ($rv === '' && !empty($result)) {
 			$entries_html = array();