Prerequisites:

1. Content type with non-requires field ezselection with at least 2 values

Steps:

1. Go to Content > Content Structure > Create
2. Select content type from prerequisites

   Already here you can see, that select is set to first value, and cannot be cleared

3. Select another option
4. Click X right to selected option

   Option is set back to first value from list

Actual result:

You cannot set selection field to empty, despite it's non-required.

Expected result:

Selection field is empty by default, and you can set it to empty.