Add add_empty_filtering_choice util for empty-value filtering

netbox/dcim/choices.py

@@ -1483,7 +1483,6 @@ class CableTypeChoices(ChoiceSet):
     TYPE_AOC = 'aoc'
     TYPE_POWER = 'power'
     TYPE_USB = 'usb'
-    TYPE_EMPTY = settings.FILTERS_NULL_CHOICE_VALUE
 
     CHOICES = (
         (
@@ -1520,7 +1519,6 @@ class CableTypeChoices(ChoiceSet):
             _('Other'), (
                 (TYPE_USB, _('USB')),
                 (TYPE_POWER, _('Power')),
-                (settings.FILTERS_NULL_CHOICE_VALUE, _('(unset)')),
             )
         )
     )

netbox/dcim/forms/filtersets.py

@@ -10,7 +10,7 @@ from ipam.models import ASN, VRF
 from netbox.forms import NetBoxModelFilterSetForm
 from tenancy.forms import ContactModelFilterForm, TenancyFilterForm
 from users.models import User
-from utilities.forms import BOOLEAN_WITH_BLANK_CHOICES, FilterForm, add_blank_choice
+from utilities.forms import BOOLEAN_WITH_BLANK_CHOICES, FilterForm, add_blank_choice, add_empty_filtering_choice
 from utilities.forms.fields import ColorField, DynamicModelMultipleChoiceField, TagFilterField
 from utilities.forms.rendering import FieldSet
 from utilities.forms.widgets import NumberWithOptions
@@ -1052,7 +1052,7 @@ class CableFilterForm(TenancyFilterForm, NetBoxModelFilterSetForm):
     )
     type = forms.MultipleChoiceField(
         label=_('Type'),
-        choices=add_blank_choice(CableTypeChoices),
+        choices=add_empty_filtering_choice(add_blank_choice(CableTypeChoices)),
         required=False
     )
     status = forms.MultipleChoiceField(

netbox/utilities/forms/utils.py

@@ -1,6 +1,7 @@
 import re
 
 from django import forms
+from django.conf import settings
 from django.forms.models import fields_for_model
 from django.utils.translation import gettext as _
 
@@ -10,6 +11,7 @@ from .constants import *
 
 __all__ = (
     'add_blank_choice',
+    'add_empty_filtering_choice',
     'expand_alphanumeric_pattern',
     'expand_ipaddress_pattern',
     'form_from_model',
@@ -189,6 +191,14 @@ def add_blank_choice(choices):
     return ((None, '---------'),) + tuple(choices)
 
 
+def add_empty_filtering_choice(choices):
+    """
+    Add an empty (null) choice to the end of a choices list, to be used in filtering classes
+    such as NullableMultipleChoiceFilter to match on an empty value.
+    """
+    return tuple(choices) + ((settings.FILTERS_NULL_CHOICE_VALUE, '(unset)'),)
+
+
 def form_from_model(model, fields):
     """
     Return a Form class with the specified fields derived from a model. This is useful when we need a form to be used