Empty Flow Names cause inconsistencies

[01Parzival10]

For preserving control flows (for example for converting PCM 2 DFD) we generally use Flows with empty names. When converting these Assignments to the WebEditor representation the empty flow names cause issues, since the WebEditor works name and not ID based. This means when going from PCM -> DFD -> JSON we create invalid assignments in the WebEditor and cannot convert back into DFD without fixing it.

Possible Solutions:

1. Dont add empty flow names - use something like "control" (Would cause issues if there is already a DFV called control present)
2. Create Logic in the WebEditor for handling one empty flow per node by addressing it with (empty / control ....) (This would cause issues if 2 empty flows are present, however we forbid 2 flows with the same name anyway)

[sebinside]

~

[01Parzival10]

The converter side has been adjusted in DataFlowAnalysis/Converter#24

For the Editor: Additionally to allowing ~ as flow names in assignments, we also need to forbid empty flow names. These can not be interpreted by Converter, since it cant know whether no or the empty flow is referenced.