ui: dropdown - label is floating when empty multiSelect is enabled and value == []

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

The label is floating for the multiSelect Stark Dropdown when empty value (value === []).

Expected behavior

When there is no value, the label should not be floating.

What is the motivation / use case for changing the behavior?

bug fixing

Environment

Angular version: X.Y.Z
Stark version: 10.0.0-beta.8


Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: