fix destructuring of empty string

packages/babel-helpers/src/helpers.js

@@ -903,7 +903,7 @@ helpers.iterableToArrayLimit = helper("7.0.0-beta.0")`
     // _i = _iterator
     // _s = _step
 
-    var _i = arr && (typeof Symbol !== "undefined" && arr[Symbol.iterator] || arr["@@iterator"]);
+    var _i = arr == null? null: (typeof Symbol !== "undefined" && arr[Symbol.iterator] || arr["@@iterator"]);
     if (_i == null) return;
 
     var _arr = [];

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/empty-string/exec.js

@@ -0,0 +1,3 @@
+let [a] = '';
+
+expect(undefined).toBe(a)

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/empty-string/input.js

@@ -0,0 +1 @@
+let [a] = '';

packages/babel-plugin-transform-destructuring/test/fixtures/destructuring/empty-string/output.js

@@ -0,0 +1,3 @@
+var _ref = '',
+    _ref2 = babelHelpers.slicedToArray(_ref, 1),
+    a = _ref2[0];