No null option is present for "RFC2317 Parent Managed" in Zone bulk edit

[peteeckel]

Versions
NetBox Version: 3.7.4
NetBox DNS Version: 0.22.6
Python Version: 3.11.5

Describe the bug
In the bulk edit form for zones, it is not possible to leave the "RFC2317 Parent Managed" property empty, resulting in the value not being changed.

To Reproduce
Select a number of zones and click on "Edit Selected"

Expected result
The "RFC2317 Parent Managed" has a null option, resolving to "leave the field alone".

Actual result
There is a boolean field for "RFC2317 Parent Managed", resulting in the setting being changed for all zones.