Ensure that the empty dictionary won't be accidentally modified

Currently there's nothing that prevents modification of the `Dict.empty` primitive, which obviously needs to be *truly* empty to prevent any future (hard to find) bugs.

src/core/primitives.js

@@ -171,7 +171,14 @@ var Dict = (function DictClosure() {
     },
   };
 
-  Dict.empty = new Dict(null);
+  Dict.empty = (function () {
+    const emptyDict = new Dict(null);
+
+    emptyDict.set = (key, value) => {
+      unreachable("Should not call `set` on the empty dictionary.");
+    };
+    return emptyDict;
+  })();
 
   Dict.merge = function ({ xref, dictArray, mergeSubDicts = false }) {
     const mergedDict = new Dict(xref);