A few problems were found in the fix for bug 43668:
1) Comparison of the YEAR column with NULL always returned TRUE;
2) Comparison of the YEAR column with constants always returned
   unpredictable result;
3) Unnecessary conversion warnings when comparing a non-integer
   constant with a NULL value in the YEAR column;

The problems described above have been resolved with an
exception: zero (i.e. invalid) YEAR column value comparison
with 00 or 2000 still fail (it is not a regression and it was
not a regression), so MIN/MAX on YEAR column containing zero
value still fail.


mysql-test/r/type_year.result:
  Test case for bug #49480.
mysql-test/t/type_year.test:
  Test case for bug #49480.
sql/item_cmpfunc.cc:
  - The get_year_value() function has been modified to make its
    return value compatible with the get_datetime_value() return
    value (i.e. to convert numeric values into the YYYY0000000000
    (YYYY-00-00 00:00:00) form.
  
  - The Arg_comparator::set_cmp_func method has been modified to
    use the get_year_value function if get_datetime_value() is not
    applicable.
    From now only 2 cases have a special processing there:
    * both comparing items have MYSQL_TYPE_YEAR field type
            or
    * one item have is MYSQL_TYPE_YEAR and other one is
      is_datetime()-compliant.
  
  - New helper function try_year_cmp_func() has been
    added for the better code readability to call from
    Arg_comparator::set_cmp_func().
  
  - The Arg_comparator::compare_year method has been removed
    since get_year_value() is compatible with the old
    Arg_comparator::compare_datetime method that doesn't have
    problems #1-#3 (see whole patch entry commentary).
sql/item_cmpfunc.h:
  - New helper function try_year_cmp_func() has been
    added for the better code readability to call from
    Arg_comparator::set_cmp_func().
  
  - Unnecessary Arg_comparator::year_as_datetime and
    Arg_comparator::compare_year() declarations have been
    removed.