navd: handle maxspeed being none (#24871)
navd: handle maxspeed being none (#24871)

* navd: handle maxspeed being none

* none is encoded like this
commaai/openpilot,commaai/openpilot,commaai/openpilot,commaai/openpilot,commaai/openpilot,commaai/openpilot