os-release: Fix VERSION_CODENAME in case it is empty

os-release: Fix VERSION_CODENAME in case it is empty

If DISTRO_CODENAME was not set, VERSION_CODENAME field was populated
with unparsed string. This resulted in the following line in os-release
file:

  VERSION_CODENAME="${DISTRO_CODENAME}"

According to systemd documentation, this field is optional.
Fix the problem by setting VERSION_CODENAME conditionally, only if
DISTRO_CODENAME was set.

Signed-off-by: Daniel Semkowicz <dse@thaumatec.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>