fix(css): return undefined if there is no className

Author: logonoff

packages/react-styles/src/index.ts

@@ -2,7 +2,7 @@
  *
  * @param {any} args list of objects, string, or arrays to reduce
  */
-export function css(...args: any): string {
+export function css(...args: any): string | undefined {
   // Adapted from https://github.com/JedWatson/classnames/blob/master/index.js
   const classes = [] as string[];
   const hasOwn = {}.hasOwnProperty;
@@ -26,5 +26,5 @@ export function css(...args: any): string {
     }
   });
 
-  return classes.join(' ');
+  return classes.join(' ').trim() || undefined;
 }