nuxeo-directory-suggestion should not try to run automation when to operation=""

• Take any edit or create layout
• Add a nuxeo-directory-suggestion:

<nuxeo-directory-suggestion value="{{document.properties.dc:course}}" label="Test" directory-name="wedonotcare" role="widget" min-chars="0" operation=""></nuxeo-directory-suggestion>

Notice operation=""

Deploy and test:

• Display the Web inspector, the console
• Click the nuxeo-directory-suggestion
• See we have an error, a 405: POST http://localhost:8080/nuxeo/api/v1/automation/ 405

So 2 bugs here:
1. Designer should remove empty property
2. WebUI should not try to connect when the operation is ""

Workaround: Go to the source, remove the operation=""