obs: srcpkg: support having no git tags

Instead of failing when a project has no git-version-gen script, and no
git tags, use 0.0.0 as version. This is needed for the new osmo-epdg
project, which doesn't have tags yet.

Change-Id: I2f0f409feb96611095dfa511a33ea730e5bbf3e4
1 file changed