[light] Fix gamma LUT quantizing small brightness to zero

The gamma LUT refactor (#14123) introduced a regression where small
brightness values (e.g. 1%) get quantized to exactly 0.0 because
the uint16 LUT entries round down to 0 for indices 1-3 with the
default gamma of 2.8.

This breaks zero_means_zero: true in FloatOutput because the
min_power scaling is skipped when state == 0.0, causing LEDs to
turn off completely instead of respecting the configured min_power.

Fix by clamping non-zero LUT entries to a minimum of 1, preserving
the invariant that non-zero input always produces non-zero output.

Fixes #15055

esphome/components/light/__init__.py

@@ -88,7 +88,15 @@ def _get_or_create_gamma_table(gamma_correct):
 
     if gamma_correct > 0:
         forward = [
-            HexInt(min(65535, int(round((i / 255.0) ** gamma_correct * 65535))))
+            # Clamp non-zero indices to minimum of 1 to preserve the invariant
+            # that non-zero input always produces non-zero output. Without this,
+            # small brightness values (e.g. 1%) get quantized to exactly 0.0,
+            # which breaks zero_means_zero logic in FloatOutput.
+            HexInt(
+                max(1, min(65535, int(round((i / 255.0) ** gamma_correct * 65535))))
+                if i > 0
+                else HexInt(0)
+            )
             for i in range(256)
         ]
     else: