value.  Document this change in behaviour, and make all callers invoke
the function right after dtuple_create().

dict_create_sys_fields_tuple(): Add a missing "break" statement to the loop
that checks if there are any column prefixes in the index.

row_get_prebuilt_insert_row(): Do not set the fields to the SQL NULL value,
now that dict_table_copy_types() takes care of it.