Add CSS to ensure there is a drop target for empty group

[Steve-Mcl]

closes #1228

Description

Simple to fix, stupid hard to find!

Adds CSS to pseudo classes :empty and :empty::before to generate an "empty state" that is essentially a drop target

Demo

Related Issue(s)

#1228

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[bartbutenaers]

@Steve-Mcl,
How on earth did you find that one...
Nice catch, and really appreciated!!

[Steve-Mcl]

Just noticed, the same issue exists for dropping groups onto a page.

#1228 (comment)

[joepavitt]

Just noticed, the same issue exists for dropping groups onto a page.

Will review this now, and we can hit that into a follow up

[joepavitt]

Cleaned up the CSS a little: