Fix: EEVEE Compiler Issue Emission Shader

Due to recent changes the local variables where not explicitly cast, that failed
when compiling on Metal.

Pull Request: https://projects.blender.org/blender/blender/pulls/115463

source/blender/gpu/shaders/material/gpu_shader_material_emission.glsl

@@ -4,8 +4,8 @@
 
 void node_emission(vec4 color, float strength, float weight, out Closure result)
 {
-  color = max(color, vec4(0));
-  strength = max(strength, 0);
+  color = max(color, vec4(0.0));
+  strength = max(strength, 0.0);
 
   ClosureEmission emission_data;
   emission_data.weight = weight;