Better error message on when the dataframe is empty for margins=True

[show981111]

For the issue #60768

In crosstab function, if the result dataframe is empty and margins=True, then we get index-out-of-bounds since the dataframe is empty. Instead of this, we can throw an error message indicating the result dataframe is empty so that the user can have a better idea.

• closes BUG: crosstab(aggfunc='skew') raises IndexError: single positional indexer is out-of-bounds #60768 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

[natmokval]

Thanks for improving the error message.
I suggest leaving the error type IndexError as it is and re-raising with a different error message.

We can put this line table.iloc[-1, :].name, where we get the problem with indexing, in a try block and then re-raise:

except IndexError as err:
    raise IndexError("Can't get margins since the result dataframe is empty") from err

[natmokval]

@show981111, could you please add a test for the new error message?