Fix #12983 bulk edit of M2M fields when nothing is selected

Partially revert 41c92483a0 to restore
bulk edit with m2m fields. The m2m cleaned_data yields a empty queryset
when nothing is selected. By setting the m2m relation unless set null is
checked even when nothing is selected the m2m relation is always
cleared.

This commit only sets the m2m relation when a selection is made.

netbox/netbox/views/generic/bulk_views.py

@@ -551,7 +551,7 @@ class BulkEditView(GetReturnURLMixin, BaseMultiObjectView):
             for name, m2m_field in m2m_fields.items():
                 if name in form.nullable_fields and name in nullified_fields:
                     getattr(obj, name).clear()
-                else:
+                elif form.cleaned_data[name]:
                     getattr(obj, name).set(form.cleaned_data[name])
 
             # Add/remove tags