Fix: Light flash not restoring previous LightState (#2383)

* Update light state when transformer has finished

* Revert writing direct to output

* Correct handling of zero-length light transformers

* Allow transformers to handle zero-length transitions, and check more boundary conditions when transitioning back to start state

* Removed log.h

* Fixed race condition between LightFlashTransformer.apply() and is_finished()

* clang-format

* Step progress from 0.0f to 1.0f at t=start_time for zero-length transforms to avoid divide-by-zero

esphome/components/light/addressable_light.cpp

@@ -79,7 +79,7 @@ optional<LightColorValues> AddressableLightTransformer::apply() {
   // dynamically-calculated alpha values to match the look.
 
   float denom = (1.0f - smoothed_progress);
-  float alpha = denom == 0.0f ? 0.0f : (smoothed_progress - this->last_transition_progress_) / denom;
+  float alpha = denom == 0.0f ? 1.0f : (smoothed_progress - this->last_transition_progress_) / denom;
 
   // We need to use a low-resolution alpha here which makes the transition set in only after ~half of the length
   // We solve this by accumulating the fractional part of the alpha over time.