Evaluate.php: add a check for better error detection.

extension/qfq/qfq/Evaluate.php

@@ -212,7 +212,7 @@ class Evaluate {
         $dbIndex = $this->dbIndex;
 
         // Check if the $token starts with '[<int>]...' - yes: open the necessary database.
-        if ($token[0] === '[') {
+        if (strlen($token) > 2 && $token[0] === '[') {
             if ($token[2] !== ']') {
                 throw new UserFormException("Missing token ']' in '$token' on position 3", ERROR_TOKEN_MISSING);
             }
@@ -224,6 +224,10 @@ class Evaluate {
             }
         }
 
+        if ($token === '') {
+            return '';
+        }
+
         if ($token[0] === '!') {
             $token = trim(substr($token, 1));
             $sqlMode = ROW_REGULAR;