refactor: simplify validations

apps/server/src/api-data/assets/assets.validation.ts

@@ -2,7 +2,7 @@ import { Request, Response, NextFunction } from 'express';
 import { body, validationResult } from 'express-validator';
 
 export const validatePostCss = [
-  body('css').exists().isString().trim(),
+  body('css').isString().trim(),
 
   (req: Request, res: Response, next: NextFunction) => {
     const errors = validationResult(req);