fix: raise AttributeError instead of returning None on __getattr__ (#329)

The` __getattr__` method should always raise an `AttributeError` if the
value does not exist instead of returning `None`. The existing
implementation causes issues when converting to a Pandas `DataFrame`.

This error was introduced in the 0.5.0 release.

Author: tdstein

src/posit/connect/resources.py

@@ -45,7 +45,7 @@ def __getattr__(self, name):
                 stacklevel=2,
             )
             return self[name]
-        return None
+        raise AttributeError(f"'{self.__class__.__name__}' object has no attribute '{name}'")
 
     def update(self, *args, **kwargs):
         super().update(*args, **kwargs)