`IcingaFormDecorator::assembleLabel()`: Why is ` ` rendered when the `label` is not set?

[sukhwinder33445]

The   was introduced by the ipl\Web\FormDecorator (legacy decorator : commit e5e8c20), without any clear explanation. To keep the behavior consistent, we added it to the new LabelDecorator as well.
The only review comments regarding this change that I could find are the following:

1. Introduce ScheduleElement #101 (comment)
2. Introduce ScheduleElement #101 (comment)

[sukhwinder33445]

@yhabteab, do you remember the reason?

[yhabteab]

@yhabteab, do you remember the reason?

I must say, I've absolutely no idea why I did that either.