`wrap_many_related_manager_add` tries to update `through_defaults`, assuming it is a dict. It is None by default

Python 3.10.6
Django 4.0.9
django-multitenant 3.2

```python
def wrap_many_related_manager_add(many_related_manager_add):
...
def add(self, *objs, through_defaults=None):
...
if hasattr(self.through, "tenant_field") and get_current_tenant():
through_defaults[
get_tenant_column(self.through)
] = get_current_tenant_value()
```

```
File ".../django/db/models/fields/related_descriptors.py", line 1130, in set
self.add(*new_objs, through_defaults=through_defaults)
File ".../django_multitenant/mixins.py", line 42, in add
through_defaults[
TypeError: 'NoneType' object does not support item assignment
```