Add neutralModel to empty.model.yml so it is defined somewhere

Author: owen-mc

go/ql/lib/ext/empty.model.yml

@@ -13,3 +13,7 @@ extensions:
       pack: codeql/go-all
       extensible: summaryModel
     data: []
+  - addsTo:
+      pack: codeql/go-all
+      extensible: neutralModel
+    data: []