THD::copy_db_to(): Always return true if the output parameter
was left uninitialized. This fixes a regression that was caused
by commit 7d0d934c (MDEV-16473).

MariaDB Server 10.3 and later were unaffected by this bug
thanks to commit a7e352b5.

Possibly this bug only affects mysql_list_fields()
in the Embedded Server (libmysqld).

This bug was found by GCC 11.2.0 in CMAKE_BUILD_TYPE=RelWithDebInfo.