require string type for patch op/path/from fields

tests/json_patch_tests.json

@@ -554,6 +554,12 @@
        "doc": {"foo": "bar"},
        "patch": [{"op": "move", "from": null, "path": "/foo"}],
        "error": "a null from field should fail rather than crash"
+    },
+
+    { "comment": "null path field must be rejected, not dereferenced",
+       "doc": {"foo": "bar"},
+       "patch": [{"op": "remove", "path": null}],
+       "error": "a null path field should fail rather than crash"
     }
 
 ]