Prevent nuxeo-selectivity from trying to get suggestion when the operation is an empty string

• In Studio Modeler create a simple vocabulary, MyVoc, with 1-2 values (we don't care, not the issue).
• Have a custom document type, MyDoc, with one field, mydoc:mystring, type directory, linked to MyVoc.
• Go to Designer and generate the layout for edit mode (for instance), then click the field and add an operation to the operation parameter (whatever, we don't care)
• Save
• Reload the page, and now, remove the text you put for the operation parameter.
• If you look at the code you will see <nuxeo-directory-suggestion ... operation="" ...>, where you have an empty string for the 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

Issues:
1. WebUI should not try to connect when the operation is an empty string
2. Designer could try to assert if an empty string is a possible value or not (not possible at the moment)

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