gst_plugin_version_helper::get_info() panics without initial commit in git repository

cargo new by default initializes a new git repository with no commits (i.e., .git/refs/heads/ is empty). This causes gst_plugin_version_helper::get_info() to panic with "Couldn't find last commit" when building in a brand-new project.

Would prefer that it falls back to what is specified in the documentation when unable to extract the desired info: COMMIT_ID is set to the string UNKNOWN and BUILD_REL_DATE to the current date.