unit-test: meson: allow missing meson-options.txt
unit-test: meson: allow missing meson-options.txt

meson-options.txt is not required, so avoid an ENOENT exception and
proceed with an empty set of options when meson-options.txt doesn't
exist.

Tested: Sucessfully ran run-unit-test-docker.sh against
https://gerrit.openbmc-project.xyz/c/openbmc/obmc-ikvm/+/19893/1

Change-Id: Ie063df12aa511f43f347db277858075bcecad024
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts