followup to fix for bug #1790 BIT_AND() result in GROUP BY different when

SQL_BIG_RESULT used": now BIT_AND() always returns 18446744073709551615
if no rows were found.
This patch also fixes bug #1972: "BIT_AND() and BIT_OR() still return 
a *signed* 64bit value"


mysql-test/r/func_group.result:
  followup to fix for bug #1790 BIT_AND() result in GROUP BY different when 
  SQL_BIG_RESULT used": test results fixed
  bug #1972 "BIT_AND() and BIT_OR() still return a *signed* 64bit value":
  results of new tests
mysql-test/t/func_group.test:
  added tests for bug #1972 "BIT_AND() and BIT_OR() still return a 
  *signed* 64bit value"