fix spot dir nan bug (bevyengine#6167)

# Objective

fix error with pbr shader's spotlight direction calculation when direction.y ~= 0

## Solution

in pbr_lighting.wgsl, clamp `1-x^2-z^2` to `>= 0` so that we can safely `sqrt` it

crates/bevy_pbr/src/render/pbr_lighting.wgsl

@@ -248,7 +248,7 @@ fn spot_light(
 
     // reconstruct spot dir from x/z and y-direction flag
     var spot_dir = vec3<f32>(light.light_custom_data.x, 0.0, light.light_custom_data.y);
-    spot_dir.y = sqrt(1.0 - spot_dir.x * spot_dir.x - spot_dir.z * spot_dir.z);
+    spot_dir.y = sqrt(max(0.0, 1.0 - spot_dir.x * spot_dir.x - spot_dir.z * spot_dir.z));
     if ((light.flags & POINT_LIGHT_FLAGS_SPOT_LIGHT_Y_NEGATIVE) != 0u) {
         spot_dir.y = -spot_dir.y;
     }