Item_func_group_concat::fix_fields() set maybe_null flag to 0, and set it to
1 only if some of it's arguments may be null. When used in subquery in tmp 
table created field which can't be null. When no data retireved result field
have to be set to null and error mentioned in bug report occurs. Also this 
bug can occur if selecting from not null field in empty table.

Function group_concat now marked maybe_null from the very beginning not only
if some of it's argument may be null.