Fix 'Undefined index: _stats' notice in bug API

This is a regression introduced by a change in filter API by commit
5458f70. In filter_cache_result(), the
2nd param to bug_cache_database_result() was changed from 'false' to
default (null). Consequently the '_stats' element was no longer present,
triggering the notice.

Problem solved by relying on array_key_exists() instead of is_null() to
check for the '_stats' array element.

Fixes #17494

core/bug_api.php

@@ -1485,13 +1485,8 @@ function bug_get_bugnote_stats( $p_bug_id ) {
 	global $g_cache_bug;
 	$c_bug_id = (int)$p_bug_id;
 
-	if( !is_null( $g_cache_bug[$c_bug_id]['_stats'] ) ) {
-		if( $g_cache_bug[$c_bug_id]['_stats'] === false ) {
-			return false;
-		} else {
-			$t_stats = $g_cache_bug[$c_bug_id]['_stats'];
-		}
-		return $t_stats;
+	if( array_key_exists( '_stats', $g_cache_bug[$c_bug_id] ) ) {
+		return $g_cache_bug[$c_bug_id]['_stats'];
 	}
 
 	$t_bugnote_table = db_get_table( 'bugnote' );