fix error on phrase search with no results. fixes dokuwiki#4355

When no pages for a phrase can be found, eg. because the index is still
empty, end($stack) returns false instead of an array, breaking the
following loop.

Author: splitbrain

inc/fulltext.php

@@ -95,9 +95,9 @@ function _ft_pageSearch(&$data)
             case 'P-:': // phrase
                 $phrase = substr($token, 3);
                 // since phrases are always parsed as ((W1)(W2)...(P)),
-                // the end($stack) always points the pages that contain
+                // the end($stack) always points at the pages that contain
                 // all words in this phrase
-                $pages  = end($stack);
+                $pages  = $stack ? end($stack) : [];
                 $pages_matched = [];
                 foreach (array_keys($pages) as $id) {
                     $evdata = [