Made the search bar on emoji menu sticky

## What does this MR do?

When scrolling down the emoji menu, the search bar disappears. For better UX, the search bar no stays at the top when scrolling.

## Screenshots (if relevant)

![emoji-sticky](/uploads/a5b4773547d3d67342ddcfc07c8f1568/emoji-sticky.gif)

See merge request !4743

Avoid a TypeError when initializing MergeRequest JS class with no arg

## What does this MR do?

Avoid a TypeError when initializing MergeRequest JS class with no arg.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

Without this sane default you would get the following error when you
tried to instantiate a new MergeRequest object with no argument (i.e.
`new MergeRequest();`):
    
    TypeError: undefined is not an object (evaluating 'this.opts.action')

## What are the relevant issue numbers?

Fixes #13525.

## Does this MR meet the acceptance criteria?

- [x] No CHANGELOG since it's a trivial internal change
- [x] Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4667