Fix crash on device without nits-backlight mapping

When there is no nits mapping, we sent NaN for the
sdr backlight value, which is invalid. Instead, send
the same (valid) backlight value for normal and SDR
brightnesses.

Bug: 186895034
Test: Manually verify that a device without nit-backlight
      mapping can still boot normally.
Change-Id: Ie6786fa17776674d914e66727f324368831f6761