Creation of an alert without a field rule type breaks the list of alert rules

[c8y3]

If an alert with a field rule that does not have a rule type is created then the list of rules can not be displayed anymore.
To reproduce:

• got to the new alert rule creation form
• fill the title, and first field rule field name and value, but not the type (matches exactly, does not match...)
• the save button is now greyed out, click on it
• the list of alerts can not be displayed anymore

Server side, the POST request should not create an invalid alert in database.
Interface side, the save button should not be available before the type is correctly filled in.