Revert removal of compare nesting

Since we want the user to be able to pass a negative comparison function, the extra layer of wrapping is now needed

src/core/matchers/toBeNull.js

@@ -1,10 +1,12 @@
 getJasmineRequireObj().toBeNull = function() {
 
   function toBeNull() {
-    return function(actual) {
-      return {
-        pass: actual === null
-      };
+    return {
+      compare: function(actual) {
+        return {
+          pass: actual === null
+        };
+      }
     };
   }