Clear dropdown selection fix

[bartbutenaers]

Description

According to the documentation, the current selection in ui-dropdown should be cleared when msg.payload = [].
However when "Allow Multiple" is deselected in the node's config screen, then the selection is not cleared.

This PR fixes that:

[{"id":"79e59c62a86947fc","type":"ui-dropdown","z":"f624e6564f974a57","group":"65805ec358e05ea4","name":"Single select option","label":"Select Option:","tooltip":"","order":0,"width":0,"height":0,"passthru":false,"multiple":false,"options":[{"label":"Option A","value":"A","type":"str"},{"label":"Option B","value":"B","type":"str"},{"label":"Option C","value":"C","type":"str"}],"payload":"","topic":"topic","topicType":"msg","className":"","x":810,"y":340,"wires":[[]]},{"id":"adad1a977d52d7ee","type":"inject","z":"f624e6564f974a57","name":"Select B","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[\"B\"]","payloadType":"json","x":600,"y":340,"wires":[["79e59c62a86947fc"]]},{"id":"90f3b037c32b470f","type":"inject","z":"f624e6564f974a57","name":"Clear","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[]","payloadType":"json","x":610,"y":380,"wires":[["79e59c62a86947fc"]]},{"id":"184aa25927b97082","type":"ui-dropdown","z":"f624e6564f974a57","group":"65805ec358e05ea4","name":"Multi select option","label":"Select Option:","tooltip":"","order":0,"width":0,"height":0,"passthru":false,"multiple":true,"options":[{"label":"Option A","value":"A","type":"str"},{"label":"Option B","value":"B","type":"str"},{"label":"Option C","value":"C","type":"str"}],"payload":"","topic":"topic","topicType":"msg","className":"","x":810,"y":460,"wires":[[]]},{"id":"d5b1176aa3646600","type":"inject","z":"f624e6564f974a57","name":"Select B","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[\"B\"]","payloadType":"json","x":600,"y":460,"wires":[["184aa25927b97082"]]},{"id":"c8b98416094457e6","type":"inject","z":"f624e6564f974a57","name":"Clear","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[]","payloadType":"json","x":610,"y":500,"wires":[["184aa25927b97082"]]},{"id":"65805ec358e05ea4","type":"ui-group","name":"Heatmap demo","page":"0a90f52e8742b298","width":"10","height":"10","order":-1,"showTitle":true,"className":"","visible":"true","disabled":"false"},{"id":"0a90f52e8742b298","type":"ui-page","name":"Heatmap","ui":"be29745a6e568f30","path":"/page2","icon":"home","layout":"grid","theme":"092547d34959327c","order":-1,"className":"","visible":"true","disabled":"false"},{"id":"be29745a6e568f30","type":"ui-base","name":"UI Name","path":"/dashboard","includeClientData":true,"acceptsClientConfig":["ui-notification","ui-control"],"showPathInSidebar":false},{"id":"092547d34959327c","type":"ui-theme","name":"Theme Name","colors":{"surface":"#ffffff","primary":"#0094ce","bgPage":"#eeeeee","groupBg":"#ffffff","groupOutline":"#cccccc"}}]

Related Issue(s)

Closes #756

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

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

[bartbutenaers]

Not quite sure in what way the 2 failed tests (form & control) could be related to my change ;-(