I've decided shader globals shouldn't be initialized in their respective shaders because their initial values are specified in the project config.

shaders/sky.gdshader

@@ -37,11 +37,11 @@ uniform sampler2D cloud_map:
 uniform sampler2D star_map:
 	source_color, filter_linear, repeat_enable, hint_default_black;
 /// Interpolates between brightest time of day and darkest time of night.
-global uniform float night = 0.0;
+global uniform float night;
 /// Interpolates quadratically between clear sky and cloudy sky.
-global uniform float overcast = 0.25;
+global uniform float overcast;
 /// Interpolates quadratically between no wind and maximum wind.
-global uniform float wind = 0.25;
+global uniform float wind;
 
 /// Computes cloud RGBA at given UV.
 vec4 clouds(vec2 uv) {