fix(clickhouse): more explicitly disallow null structs

[NickCrews]

I remove the docstring because it is obvious from the type signature,
and the docstring was wrong. The superclass has the correct docstring though,
just inherit that.

Part of #8666

[cpcloud]

FYI docstrings are not inherited for overridden methods (though IPython helps us out here), but I still don't think we need it here :)

In [1]: class Base:
   ...:     def run(self):
   ...:         """Run some code"""
   ...:

In [2]: class Child(Base):
   ...:     def run(self): ...
   ...:

In [3]: Child.run?
Signature: Child.run(self)
Docstring: Run some code
File:      ~/src/github.com/ibis/<ipython-input-2-114bdfe3357a>
Type:      function

In [4]: Child.run.__doc__

In [5]: Base.run.__doc__
Out[5]: 'Run some code'

[cpcloud]

Crushing it!

[NickCrews]

ahh, thanks for the point re docstrings. VSCode is doing the same thing as ipython and shows the parent class's docstring when I hover over the method, but I see now that is being misleading as to what is actually happening :)