check_license: Work if third_party doesn't exist.
check_license: Work if third_party doesn't exist.

Signed-off-by: Tim 'mithro' Ansell <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@mith.ro>

SymbiFlow/python-symbiflow-v2x,SymbiFlow/python-symbiflow-v2x