db/ColumnSchemaBuilder::buildCommentString() returns empty string by default

framework/db/mysql/ColumnSchemaBuilder.php

@@ -43,6 +43,14 @@ class ColumnSchemaBuilder extends AbstractColumnSchemaBuilder
         return $this->isFirst ? ' FIRST' : '';
     }
 
+    /**
+     * @inheritdoc
+     */
+    protected function buildCommentString()
+    {
+        return $this->comment !== null ? " COMMENT " . $this->db->quoteValue($this->comment) : '';
+    }
+
     /**
      * @inheritdoc
      */