Enhance slot icon regarding "`null`" and "0%" charging states

[LucasForster]

This issue addresses the following two (not too rare) events:

• The current charging level cannot be retrieved (null): Due to const chargePercentage = Math.round((slot.stateOfCharge || 0) * 100);, this is currently treated like 0% charging level at the moment. In the past the icon dropped down: It may be enough to treat it like the next case or we could introduce a ❓emoji.
• The charging level is 0%: The bike is therefore possibly out of order: In my opinion a more alarming icon could be beneficial in this special case. An emoji like ❌ could be fitting.

[NeoLegends]

I think the red flash symbolizes an erroneous case good enough. For UI consistency I'd very much like to keep the current behavior where we use 0% for null%.

Another option I'd be kinda okay with is to just hide the bikes with a charge state of null. Since the bikes are operable in principle, however, we'd hide bikes that you could have rented out. Therefore I'm closing this for now.