[Python] Fix fatal exception caused by empty Pandas Categorical objects.

[Tishj]

This PR fixes #12364

This was caused by us accepting input that would create an ENUM of size 0, which was not expected by the rest of the system. We held an implicit assertion that the size would be over 0.

I've added the assertion and we throw a proper exception when this is attempted.

[Tishj]

@Mytherin this failure seems interesting https://github.com/duckdb/duckdb/actions/runs/9363993037/job/25775994277?pr=12370

I added an assertion earlier on so we can catch it before it causes an assertion failure when we try to create a (idx_t)-1 value inside Value::MaximumValue(type)
We could perhaps also fix this by checking if the size is 0 and then not doing -1, I'll try that, I just thought that was the wrong fix

[Mytherin]

I wonder if we should just allow zero-length enums? I can't really think of anything that would break - they just need to be NULL for every value since there is no valid value, but that is not really different from the other constraints on enums (i.e. enum values always need to be < dict_size anyway).

[Mytherin]

Thanks!

[Mytherin]

Actually looking at this again this seems wrong - Value::MaximumValue now returns an enum with value 0, but a size of 0, which is invalid/out of range. Perhaps this should return a null value instead? (Value())?