Fix inconsistent return values in the indexer and in getInfo()

inc/indexer.php

@@ -1036,7 +1036,7 @@ class Doku_Indexer {
         $fh = @fopen($cachename.'.tmp', 'w');
         if (!$fh) {
             trigger_error("Failed to write index cache", E_USER_ERROR);
-            return;
+            return $lengths;
         }
         @fwrite($fh, implode("\n", $lengths));
         @fclose($fh);

lib/plugins/syntax.php

@@ -37,6 +37,7 @@ class DokuWiki_Syntax_Plugin extends Doku_Parser_Mode {
         $info  = DOKU_PLUGIN.'/'.$parts[2].'/plugin.info.txt';
         if(@file_exists($info)) return confToHash($info);
         trigger_error('getInfo() not implemented in '.get_class($this).' and '.$info.' not found', E_USER_WARNING);
+        return array();
     }
 
     /**