Fix Geometry Nodes: Broken built-in attribute exists check

Fixes an issue in a node setup with the point separate node, where muting
a node that does nothing breaks the operation, resulting in the point
separate not copying the position attrbute to either result.

The fix is straightfoward, it looks just like a typo.

Differential Revision: https://developer.blender.org/D10379

source/blender/blenkernel/intern/attribute_access.cc

@@ -731,7 +731,7 @@ class BuiltinCustomDataLayerProvider final : public BuiltinAttributeProvider {
     if (custom_data == nullptr) {
       return false;
     }
-    const void *data = CustomData_get_layer(custom_data, data_type_);
+    const void *data = CustomData_get_layer(custom_data, stored_type_);
     return data != nullptr;
   }
 };