buildkite: Handle case of sanitycheck doing nothing

The junit-annotate step will fail if there are no sanitycheck-*.xml
files to be found which can happen if sanitycheck is run and does
nothing (for example an update to .editorconfig).

Try and create an empty sanitycheck.xml in such a case.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>