Fix issue 976

[billylanchantin]

Description

Fixes an issue with DF.new where a well placed nil could result in a panic.

Basically, the work was being done by Native.s_from_list_of_series/2 which attempted to do following:

• create a list of individual polars_core::seriess each with the same dtype
• create a final polars_core::series of type {:list, dtype} from that list

But because s_from_list_of_series/2 didn't have the final dtype information up front, it was possible for two elements of the intermediate list to have incompatible dtypes. When this happened, the final creation would panic.

Changes

This PR makes the dtype a required argument for:

• s_from_list_of_series/3
• s_from_list_of_series_as_structs/3

(Those two seemed like a pair so I made them the same.)

Links

• Closes: Panic for list of lists with nil entries #976

Discussion

I think there's an opportunity for optimization since we're now passing the expected dtype, but I didn't explore it.

[philss]

🚀