Fix Cycles area light using MIS when the spread is zero

area light with zero spread was introduced in bf18032977. Such paths can
only be sampled with NEE, so MIS should not be used.
This fixes the discrepancy when Direct Light Sampling is set to MIS or NEE.

Pull Request: https://projects.blender.org/blender/blender/pulls/118584

intern/cycles/scene/light.cpp

@@ -1331,7 +1331,7 @@ void LightManager::device_update_lights(Device *device, DeviceScene *dscene, Sce
 
       float3 dir = safe_normalize(light->get_dir());
 
-      if (light->use_mis && area != 0.0f) {
+      if (light->use_mis && area != 0.0f && light->spread > 0.0f) {
         shader_id |= SHADER_USE_MIS;
       }