The bug could cause choosing a sub-optimal execution plan for 
a single-table query if a unique index with many null keys were 
defined for the table. 
It happened because the code of the check_quick_keys function 
made an assumption that any key may occur in an unique index 
only once. Yet this is not true for keys with nulls that may 
have multiple occurrences in the index.


mysql-test/r/null_key.result:
  Fixed bug #25407
  Adjusted result after the fix.
mysql-test/r/select.result:
  Added a test case for bug #25407.
mysql-test/t/select.test:
  Added a test case for bug #25407.