by a function and column

The bugreport reveals two different bugs about grouping
on a function:

1) grouping by the TIME_TO_SEC function result caused
   a server crash or wrong results and
2) grouping by the function returning a blob caused
   an unexpected "Duplicate entry" error and wrong
   result.

Details for the 1st bug:

TIME_TO_SEC() returns NULL if its argument is invalid (empty
string for example). Thus its nullability depends not only
on the nullability of its arguments but also on their values.
Fixed by (overoptimistically) setting TIME_TO_SEC() to be
nullable despite the nullability of its arguments.

Details for the 2nd bug:

The server is unable to create indices on blobs without
explicit blob key part length. However, this fact was
ignored for blob function result fields of GROUP BY
intermediate tables.
Fixed by disabling GROUP BY index creation for blob
function result fields like regular blob fields.