Unsetting z value for static locations results in error when saving

Currently it is not possible to save an static location after removing an existing z value in the form. It currently sets the value to an empty string & that fails when saving with an 422 error (unprocessable entity).

Ways to reproduce for the moment:

• go into the form to create a new static location actions
• set x, y fields, the date & the begin contact
• set the z value to 42
• remove the 42 from the z value field
• save