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.