frontend_tests: Use let with explicit null in components test.

Even though this variable was only assigned once, it was accessed
before its initialization, so it couldn’t be converted directly to
`let` or `const`.  Use `let` with an explicit `null` to make it
clearer what’s going on and satisfy ESLint.  (Why not `undefined`?
There’s an ESLint rule against that too.)

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

frontend_tests/node_tests/components.js

@@ -124,7 +124,7 @@ run_test('basics', () => {
 
     var callback_value;
 
-    var widget;
+    let widget = null;
     widget = components.toggle({
         selected: 0,
         values: [