Test tpr19 includes a property-scoped context that includes a null value:

    "protected2": {
      "@id": "ex:protected2",
      "@context": [
        null,
        {
          "protected1": "ex:protected3"
        }
      ]
    },

When Context Processing is called during expansion of the "protected2" key, why does the first (null) context value here not trigger the error condition in Context Processing step 5.1.1?

5 For each item context in local context:

5.1 If context is null:

5.1.1 If override protected is false and active context contains any protected term definitions, an invalid context nullification has been detected and processing is aborted.