fix: TypeError: '<' not supported between instances of 'str' and 'NoneType'
I'm afraid I don't know exactly what triggers this, but in my big openapi spec this gets
triggered:
```
Traceback (most recent call last):
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/__main__.py", line 458, in main
generate(
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/__init__.py", line 492, in generate
results = parser.parse()
^^^^^^^^^^^^^^
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/parser/base.py", line 1311, in parse
self.__reuse_model(models, require_update_action_models)
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/parser/base.py", line 868, in __reuse_model
model_key = tuple(to_hashable(v) for v in (model.render(class_name="M"), model.imports))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/parser/base.py", line 868, in <genexpr>
model_key = tuple(to_hashable(v) for v in (model.render(class_name="M"), model.imports))
^^^^^^^^^^^^^^
File "/home/gjc/.cache/uv/archive-v0/zrUKlPLuGJsfxDV0Mede0/lib/python3.12/site-packages/datamodel_code_generator/parser/base.py", line 76, in to_hashable
return tuple(sorted(to_hashable(i) for i in item))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'str' and 'NoneType'
```
The objects in question look like:
```
Import(from_="typing", import_="Annotated", alias=None, reference_path=None)
```
Which are transformed into:
```
(
("alias", None),
("from_", "typing"),
("import_", "Annotated"),
("reference_path", None),
),
```
Now, those `None`s will cause `sorted()` to raise that TypeError.
The simple fix is to transform None into "": this is only used as key in a cache, so it
doesn't really matter as long as the value is predictable and hashable.