Optional enumeration fields don't allow for empty values

If a table row has been set as an optional enumeration column, the only available options are the ones in the enumeration. Setting the field as `null`/`empty` is impossible.

**Expected:**
1. In a new or existing table, add a column
- of type `enumeration`
- with the `required value` option left unchecked
2. For a new/existing entry, select one of the enumeration options for the new column
3. Click again on that value and unset the value/set it to null/empty

**Actual:**
1. In a new or existing table, add a column
- of type `enumeration`
- with the `required value` option left unchecked
2. For a new/existing entry, select one of the enumeration options for the new column
3. Clicking again on that value **there is no option to remove the current value**

**More details:**
Image showing the column creation:

Image showing the options for an entry: