This is backport of 25ad623d for 10.2.

The issue is similar to the one from MDEV-13240. Item::save_in_field()
returns an error during tmp table population in a create table from select query
as we try to save an empty string as a date value when
force_return_blank is set to true for window functions.

    MDEV-13240 Wrong warning with MAX(datetime_field) OVER (...)

    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.