HG-17 FOUC when loading comments

While loading comments we were showing two buttons `Next page` and `Previous page` before we loaded comments. There were two issues with this:
* we shouldn't show the buttons and just after that put comments list between them,
* the logic which was supposed to show/hide didn't really work.

The first issue was fixed by checking if the loaded comments from API response aren't "falsy" -- only if there are some comments in comments' array show the controls and comments to the user.

The second issue was fixed by removing type checking while setting `isFirstPage` and `isLastPage` variables.
it says nothing about it in guild lines
but there is:
https://github.com/Wikia/guidelines/blob/master/JavaScript/.jshintrc#L32

compering to null like this has it's advantage
as this is equivalent to
```js
page !== null && page !== undefined
```