Issue #2630 miscellaneous df stuff

[stefanhaerter]

Referencing #2630

I included the following changes:

• As I encountered the problem of using undefined values in DisplayValueRender and ReadableValueRender multiple times, I took the freedom to align the handling at every relevant occurrence with the drivers where it works
• AgentTicketProcess assumed in its AJAXUpdate handling that the first input of multivalue fields still has no index (DynamicField_Name vs. DynamicField_Name_0) as it has been before - I adapted the behavior to the current naming
• When adding a new reference dynamic field, EditFieldMode is undef which caused a warning in the log - for this case, I changed the value to an empty string