[6.x] Fix hidden field filters when applying to all fields

[daun]

Fix #14018 by

• Never hiding the currently applied filters
• Only hiding the input for adding new filters if all fields are already in use for filtering
• Only showing the "no filters" warning if no filters were ever available

Before

All filters are hidden once all fields are selected for filtering.

After: Scenario A: all fields already selected

If all fields of a blueprint are already selected for filtering, show no message at all.

After: Scenario B: no fields available

If no fields are available for filtering at all, show the message, but subdued.

[jasonvarga]

I appreciated leaving off the indentation change to keep the diff small for review. 👌