Don't save undefined atributes.

src/Libs/ModelMySQL.php

@@ -223,9 +223,11 @@ class ModelMySQL {
     $atts = $this->getVars();
     
     foreach ($atts as $key => $value) {
+      if (isset($value)) {
         $value = static::db()->real_escape_string($value);
         $set[]="$key='$value'";
       }
+    }
     
     $table = static::table();
     $pk = static::$primaryKey;