_HeadsetOnboardEffect.__init__ seeded per-effect defaults for any
field that was falsy, so a Static color1 of 0x000000 (black) was
treated as unset and overwritten with the white default — setting the
onboard color to black turned the LEDs white instead of off.
Switch the constructor to None-sentinel defaults: a field is seeded
from _DEFAULTS only when genuinely absent (None), so an explicit 0 is
honored. The UI's get_value() always passes explicit values, and a
fresh effect-pick seeds its RANGE widgets UI-side via _apply_id_defaults,
so animated effects still get sane defaults.
Reported by @rouderz on PR #3181.
7c7666466a