Preset widgets: Show blank, rather than magenta text, when no preset matches current state

[marktsuchida]

• pymmcore-widgets version: 0.7.2
• Python version: 3.12.4
• Operating System: macOS 14.5

If I understand correctly, in GroupPresetTableWidget and PresetsWidget (any others?), the preset name is displayed in magenta when there is no preset in the group that matches the current property values.

Because the displayed preset in this case has no particular relationship to the current state of devices (it appears to be the preset that last got displayed in black), I think this can be confusing, and propose that something neutral be displayed instead: either blank, or grayed text such as "no matching preset".

(For comparison, MMStudio displays blank in this case.)