NULLable BIGINT and INT columns in comparison

Problem: a consequence of the fix for 43668.
Some Arg_comparator inner initialization missed,
that may lead to unpredictable (wrong) comparison
results.

Fix: always properly initialize Arg_comparator
before its usage.


mysql-test/r/select.result:
  Fix for bug#49517: Inconsistent behavior while using 
  NULLable BIGINT and INT columns in comparison
    -test result.
mysql-test/t/select.test:
  Fix for bug#49517: Inconsistent behavior while using 
  NULLable BIGINT and INT columns in comparison
    -test case.
sql/item_cmpfunc.cc:
  Fix for bug#49517: Inconsistent behavior while using 
  NULLable BIGINT and INT columns in comparison
    - now all Arg_comparator::set_cmp_func() set
  Arg_comparator::set_null to ensure its proper initialization
  in all cases (by default it's set to TRUE in constructors).
sql/item_cmpfunc.h:
  Fix for bug#49517: Inconsistent behavior while using 
  NULLable BIGINT and INT columns in comparison
    - now all Arg_comparator::set_cmp_func() set
  Arg_comparator::set_null to ensure its proper initialization
  in all cases (by default it's set to TRUE in constructors).