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-destructuring/test/fixtures/destructuring/array-unpack-optimisation/expected.js

@@ -21,7 +21,7 @@ var _ref4 = [a[1], a[0]];
 a[0] = _ref4[0];
 a[1] = _ref4[1];
 
-var _ref5 = [].concat(babelHelpers.toConsumableArray(foo), [bar]),
+var _ref5 = babelHelpers.toConsumableArray(foo).concat([bar]),
     a = _ref5[0],
     b = _ref5[1];