Fix for Python 3.13

Python 3.13 introduced docstrings for None: python/cpython#117813

In Python 3.12, this is an empty string:

```
 ➜ python3.12
Python 3.12.6 (main, Sep 10 2024, 19:06:17) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> d = None
>>> d.__doc__
>>>
```

In Python 3.13, it's no longer empty:

```
 ➜ python3.13
Python 3.13.0rc2+ (heads/3.13:660baa1, Sep 10 2024, 18:57:50) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> d = None
>>> d.__doc__
'The type of the None singleton.'
>>>
```

Adding a check in the inspector that get the view description out the view function docstring to cath this edge case.

Author: browniebroke

rest_framework/schemas/inspectors.py

@@ -79,8 +79,9 @@ def get_description(self, path, method):
         view = self.view
 
         method_name = getattr(view, 'action', method.lower())
-        method_docstring = getattr(view, method_name, None).__doc__
-        if method_docstring:
+        method_func = getattr(view, method_name, None)
+        method_docstring = method_func.__doc__
+        if method_func and method_docstring:
             # An explicit docstring on the method or action.
             return self._get_description_section(view, method.lower(), formatting.dedent(smart_str(method_docstring)))
         else: