oeqa/utils/metadata.py: Fix running oe-selftest running with no distro set

This will use default values when no distribution is set.

[YOCTO #15086]

Signed-off-by: Thomas Roos <throos@amazon.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
(cherry picked from commit 888fe63b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>