[light] Skip the flash transition setter and the empty effect list (#19228)

tests/component_tests/light/config/transitions.yaml

@@ -0,0 +1,29 @@
+---
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+
+output:
+  - platform: ledc
+    id: out_a
+    pin: GPIO4
+  - platform: ledc
+    id: out_b
+    pin: GPIO5
+
+light:
+  - platform: monochromatic
+    id: plain_light
+    output: out_a
+    flash_transition_length: 0s
+  - platform: monochromatic
+    id: fancy_light
+    output: out_b
+    flash_transition_length: 500ms
+    effects:
+      - pulse:
+  - platform: monochromatic
+    id: bare_light
+    output: out_a

tests/component_tests/light/test_default_setters.py

@@ -0,0 +1,19 @@
+"""Tests that light codegen skips setters for default values."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_default_flash_length_and_empty_effects_are_not_emitted(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """A 0 ms flash transition and an empty effect list match the C++ defaults."""
+    main_cpp = generate_main(component_config_path("transitions.yaml"))
+
+    assert "plain_light->set_flash_transition_length(" not in main_cpp
+    assert "plain_light->add_effects(" not in main_cpp
+    assert "bare_light->set_flash_transition_length(" not in main_cpp
+    assert "bare_light->add_effects(" not in main_cpp
+    assert "fancy_light->set_flash_transition_length(500);" in main_cpp
+    assert "fancy_light->add_effects({" in main_cpp