Instead of Vector2() start volumetric fog at frustum_near_size to avoid INF and NaN in shaders.

(cherry picked from commit 00c2fb4a08ab83a623f388718869a818658434c5)

servers/rendering/renderer_rd/environment/fog.cpp

@@ -561,7 +561,7 @@ void Fog::volumetric_fog_update(const VolumetricFogSettings &p_settings, const P
 		if (p_cam_projection.is_orthogonal()) {
 			fog_near_size = fog_far_size;
 		} else {
-			fog_near_size = Vector2();
+			fog_near_size = frustum_near_size.max(Vector2(0.001, 0.001));
 		}
 
 		params.fog_frustum_size_begin[0] = fog_near_size.x;
@@ -1020,7 +1020,7 @@ void Fog::volumetric_fog_update(const VolumetricFogSettings &p_settings, const P
 	if (p_cam_projection.is_orthogonal()) {
 		fog_near_size = fog_far_size;
 	} else {
-		fog_near_size = Vector2();
+		fog_near_size = frustum_near_size.max(Vector2(0.001, 0.001));
 	}
 
 	params.fog_frustum_size_begin[0] = fog_near_size.x;