change sample method to work for pandas

[raisadz]

No description provided.

[MarcoGorelli]

wonderful, thanks! fancy adding a little test which just makes an assertion on the length of the output if we use this?

[raisadz]

I added __invert__() method in narwhals/expression.py and tests for .is_null() and .sample() functions. It looks like there are some issues with pandas indexing for .sample() and .drop_nulls() methods. Also, .sample() returns an eager dataframe when the native dataframe is pandas and a lazy dataframe when the native is Polars and this results in the error AttributeError: 'DataFrame' object has no attribute 'collect' for pandas and modin inputs in test test_expr_sample.

[MarcoGorelli]