frontend: fix one more traceback in builder-live.log link
    
    When SRPM build succeeds but import fails - the build is "failed" so it
    is finished, but it doesn't have result dir yet.  Trying to
    os.path.join(None, str()) throws this:
    TypeError: expected str, bytes or os.PathLike object, not NoneType
    
    Merges: #1058