fix null values in sql generator

Author: matthi4s

src/Query/Generator/SQL.php

@@ -192,8 +192,6 @@ private function generateFields(Query $query)
      */
     private function generateValue($value)
     {
-        $value = ($this->escapeFunction)($value);
-
         if (is_int($value) || is_float($value)) {
             return $value;
         }
@@ -202,6 +200,8 @@ private function generateValue($value)
             return "NULL";
         }
 
+        $value = ($this->escapeFunction)($value);
+
         return $this->stringEnclosure . $value . $this->stringEnclosure;
     }
 }