Fix #118360: Fill mask operator doesn't initialize hidden vertices

If there is no mask attribute yet and hidden vertices, the values for those
vertices wouldn't be initialized. Fix by simply initializing the entire array
when it's allocated.

source/blender/editors/sculpt_paint/paint_mask.cc

@@ -256,7 +256,7 @@ static void fill_mask_mesh(Object &object, const float value, const Span<PBVHNod
     }
   }
 
-  bke::SpanAttributeWriter<float> mask = attributes.lookup_or_add_for_write_only_span<float>(
+  bke::SpanAttributeWriter<float> mask = attributes.lookup_or_add_for_write_span<float>(
       ".sculpt_mask", bke::AttrDomain::Point);
 
   threading::EnumerableThreadSpecific<Vector<int>> all_index_data;