Update blocks.js

Author: FrciSmrci

src/blocks.js

@@ -2,7 +2,7 @@ export default function (editor, opt = {}) {
   const c = opt;
   let bm = editor.BlockManager;
   let blocks = c.blocks;
-  let stylePrefix = c.stylePrefix;
+  let stylePrefix = !!c.stylePrefix ? c.stylePrefix : 'gjs-';
   const flexGrid = c.flexGrid;
   const basicStyle = c.addBasicStyle;
   const clsRow = `${stylePrefix}row`;