Allow null for cache data, according to HAR spec.

The HAR spec allows for afterRequest and beforeRequest to be null, see
http://www.softwareishard.com/blog/har-12-spec/#cache. This has a different semantics from the fields being left out.

types/har-format/har-format-tests.ts

@@ -89,6 +89,19 @@ const testEntry: harFormat.Entry = {
     pageref: "page_1"
 };
 
+// Examples from http://www.softwareishard.com/blog/har-12-spec/#cache
+const testCacheNoInformation: harFormat.Cache = {
+};
+
+const testNoCacheAfter: harFormat.Cache = {
+  afterRequest: null
+};
+
+const testCacheNotCached: harFormat.Cache = {
+  beforeRequest: null,
+  afterRequest: null
+};
+
 const testLog: harFormat.Log = {
     version: "1.2",
     creator: testCreator,

types/har-format/index.d.ts

@@ -728,13 +728,13 @@ export interface Cache {
      *
      * Leave out this field if the information is not available.
      */
-    beforeRequest?: CacheDetails;
+    beforeRequest?: CacheDetails | null;
     /**
      * State of a cache entry after the request.
      *
      * Leave out this field if the information is not available.
      */
-    afterRequest?: CacheDetails;
+    afterRequest?: CacheDetails | null;
     /**  A comment provided by the user or the application */
     comment?: string;
 }