Treat `null` as deep-readable (same as `undefined`).

README.md

@@ -182,6 +182,19 @@ city(object, "New York") === object;
 city2(object, "New York") === object;
 // => true
   
+city(undefined);
+// => undefined
+city(null);
+// => undefined
+city({});
+// => undefined
+city({address: null});
+// => undefined
+city({address: {}});
+// => undefined
+city({address: {city: null}});
+// => null
+  
 city(undefined, "London");
 // => {address: {city: "London"}}
 city(null, "London");

src/cow-value-model.js

@@ -29,7 +29,7 @@ export const cowValueModel = (...keyDescriptions) => {
 
     return (obj, val = GET_SENTINEL) =>
         val === GET_SENTINEL ?
-            keys.reduce((x = {}, key) => x[key], obj) :
+            keys.reduce((x, key) => x == null ? undefined : x[key], obj) :
             setField(obj, 0, val);
 };