kernel/swap: Initialize dummy thread's resource pool

The resource pool of the short-lived dummy thread "stub" may be
inherited by other threads created during system initialization. This
commit initializes this resource pool to NULL or the system pool to
ensure that a well-defined resource pool propagates to other threads
that inherit it from the dummy thread.

Fixes #41482.

Signed-off-by: Berend Ozceri <berend@recogni.com>