The problem resided in Item_window_func implementation,
and it was revealed by bb-10.2-ext specific changes:
Item_window_func::save_in_field() works differently in bb-10.2-ext vs 10.2:
- 10.2 goes through val_str()
- bb-10.2-ext goes through get_date(), due to Type_handler related changes.
get_date() tries to convert empty string to DATETIME, hence the warning.
During a discussion with Vicentiu, it was decided to fix
Item_window_func::val_xxx() to return NULL
(instead of an "empty" value, such as 0 for numbers and '' for strings)
when force_return_blank is set.
When running setup fields during the final step of insert using select
the final setup_fields does not have any sum functions. Our current
condition for calling split_sum_func however would attempt to use an empty
NULL sum_func_list, if the item contained a window function.
The solution is to not perform another split_sum_func for the item
containing a window function if we do not actually have a sum_func_list.
