Fix check for existence of New Branch button on Issue page

## What does this MR do?

The condition in [`initCanCreateBranch`][initCanCreateBranch] mistakenly checks `$container` (the New Branch button) for falsy. However JQuery returns an empty array if no matching element was found, so this condition is never met.

## Why was this MR needed?

The wrong condition causes:
* `$.getJSON($container.data('path'))` to be called where `$container.data('path')` is `undefined`
* in this case `$.getJSON` uses `location.href`
* if the current page has a JSON representation, it is fetched and cached by browser and displayed the next time the page is visited (#17365)
* otherwise "Failed to check if new branch can be created" is displayed (#17264)

## What are the relevant issue numbers?

Fixes #17264 and #17365.

[initCanCreateBranch]: https://gitlab.com/gitlab-org/gitlab-ce/blob/v8.8.4/app/assets/javascripts/issue.js.coffee#L102


See merge request !4630