Fixes bulk-assign label for multiple issues not having the same labels

## What does this MR do?
Fixes a bug when bulk-assigning a label to multiple issues while the label is present in on the issues on the selection.

## Screenshots (if relevant)

**Before Bugfix**

<img src="/uploads/ad1f290bcf3930177a3a71c69cbe5325/before-bugfix.gif" width="700"/>

**After Bugfix**

<img src="/uploads/1f04d6bf027806fb13ca3773febda744/bugfix.gif" width="700"/>

## Does this MR meet the acceptance criteria?

- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)

See merge request !4602