Use default brightness/colortemp in sleep mode if not configured

[claytonjn]

If user omits sleep_colortemp the non-sleep calculated colortemp should be used. If user omits sleep_brightness the non-sleep calculated brightness should be used. If user omits both, both non-sleep values should be used (resulting in "sleep" mode being the same as "regular" mode).

[Alfiegerner]

Just to add to this - not setting sleep_colortemp but setting sleep_brightness causes following exception log spamming:

2020-07-28 21:06:47 ERROR (SyncWorker_5) [homeassistant.util.logging] Exception in update_switch when dispatching 'circadian_lighting_update': ()
Traceback (most recent call last):
  File "/home/homeassistant/.homeassistant/custom_components/circadian_lighting/switch.py", line 258, in update_switch
    self._hs_color = self.calc_hs()
  File "/home/homeassistant/.homeassistant/custom_components/circadian_lighting/switch.py", line 241, in calc_hs
    return color_xy_to_hs(*self.calc_xy())
  File "/home/homeassistant/.homeassistant/custom_components/circadian_lighting/switch.py", line 238, in calc_xy
    return color_RGB_to_xy(*self.calc_rgb())
  File "/home/homeassistant/.homeassistant/custom_components/circadian_lighting/switch.py", line 233, in calc_rgb
    return color_temperature_to_rgb(self._sleep_colortemp)
  File "/srv/homeassistant/lib/python3.8/site-packages/homeassistant/util/color.py", line 453, in color_temperature_to_rgb
    if color_temperature_kelvin < 1000:
TypeError: '<' not supported between instances of 'NoneType' and 'int'