Fix MyPy issues in AWS Sensors (#20863)

Part of apache/airflow#19891

Before:

```
root@12e9fcfb4678:/opt/airflow# mypy --namespace-packages  airflow/providers/amazon/aws/sensors
airflow/providers/amazon/aws/sensors/s3.py:182: error: Incompatible types in assignment (expression has type "function", variable has type "Callable[..., bool]")  [assignment]
            check_fn: Callable[..., bool] = self.check_fn_user if self.check_fn_user is not None else self.check_fn
                                            ^
Found 1 error in 1 file (checked 33 source files)
```

After:

```
root@12e9fcfb4678:/opt/airflow# mypy --namespace-packages  airflow/providers/amazon/aws/sensors
Success: no issues found in 33 source files
```
GitOrigin-RevId: b15027410b4a985c15b1d7b2b2a0eedf2173f416

airflow/providers/amazon/aws/sensors/s3.py

@@ -179,8 +179,9 @@ def poke(self, context: 'Context'):
         s3_objects = self.get_files(s3_hook=self.get_hook())
         if not s3_objects:
             return False
-        check_fn = self.check_fn if self.check_fn_user is None else self.check_fn_user
-        return check_fn(s3_objects)
+        if self.check_fn_user is not None:
+            return self.check_fn_user(s3_objects)
+        return self.check_fn(s3_objects)
 
     def get_files(self, s3_hook: S3Hook, delimiter: Optional[str] = '/') -> List:
         """Gets a list of files in the bucket"""