Fix #108344: default result buffer must be initially contain values

Caused by 15f464019a.

Now make sure that the `result_buffer` is properly initialized, because
it may be used when trying to blur on an unsupported attribute domain.

Pull Request: https://projects.blender.org/blender/blender/pulls/108347

source/blender/nodes/geometry/nodes/node_geo_blur_attribute.cc

@@ -423,7 +423,7 @@ class BlurAttributeFieldInput final : public bke::GeometryFieldInput {
     VArraySpan<float> neighbor_weights = evaluator.get_evaluated<float>(1);
     GArray<> buffer_b(*type_, domain_size);
 
-    GSpan result_buffer;
+    GSpan result_buffer = buffer_a.as_span();
     switch (context.type()) {
       case GeometryComponent::Type::Mesh:
         if (ELEM(context.domain(), ATTR_DOMAIN_POINT, ATTR_DOMAIN_EDGE, ATTR_DOMAIN_FACE)) {