Add test for empty version in /proc/drbd

The previous patch removed a (rather implicit) test for /proc/drbd not
containing a version line. This adds an explicit test for this case.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>