Model initialized with `skip_model_load` does not have a compiled path

Code:
```python
import coremltools as ct

m = ct.models.MLModel("", skip_model_load=True)
m.get_compiled_model_path()
```
Outputs:
```
----> 1 m.get_compiled_model_path()

File ~/miniforge3/envs/prod/lib/python3.8/site-packages/coremltools/models/model.py:528, in MLModel.get_compiled_model_path(self)
520 def get_compiled_model_path(self):
521 """
522 Returns the path for the underlying compiled ML Model.
523
(...)
526
527 """
--> 528 return self.__proxy__.get_compiled_model_path()

AttributeError: 'NoneType' object has no attribute 'get_compiled_model_path'
```
Correct behavior:
Return `None` without error.

Start in coremltools/models/model.py at MLModel.get_compiled_model_path(), then trace how skip_model_load=True leaves __proxy__ unset. Reproduce the shown call and confirm that it returns None without raising an AttributeError.

Written by the indexing model from the issue text.