No unneeded empty arrays in transform spread (#6763)

* No unneeded empty arrays in transform spread

Since Array.prototype.concat creates a new array from inputs, there's
no need to call it from a new empty array ([].concat()).

* [fixup] simplify detection of new array

packages/babel-plugin-transform-spread/README.md

@@ -15,7 +15,7 @@ var b = [...a, 'foo'];
 
 ```js
 var a = [ 'a', 'b', 'c' ];
-var b = [].concat(a, [ 'foo' ]);
+var b = a.concat([ 'foo' ]);
 ```
 
 ## Installation