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
7b18e8c744