[ios/web] Don't associate nil wk_item's.

Due to an unrelated issue, it's possible for about:// urls to be
written as an empty url.  Because an empty forward or backwards item
will return an empty URL, it was possible to associate to a nil
WKBackForwardItem and crash.

Bug: 1005211
Change-Id: I2becd93a8d35cde0f1a68fc2ac3da6853c023392
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810803
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697892}

ios/web/navigation/navigation_manager_impl_unittest.mm

@@ -2633,6 +2633,19 @@ TEST_P(NavigationManagerTest, CommitNilPendingItem) {
             navigation_manager()->GetLastCommittedItem()->GetURL());
 }
 
+// Tests that NavigationManagerImpl::CommitPendingItem() for an invalid URL
+// doesn't crash.
+TEST_P(NavigationManagerTest, CommitEmptyPendingItem) {
+  [mock_wk_list_ setCurrentURL:@"http://www.url.com/1"
+                  backListURLs:nil
+               forwardListURLs:nil];
+
+  // Call CommitPendingItem() with a valid pending item.
+  auto item = std::make_unique<web::NavigationItemImpl>();
+  item->SetURL(GURL::EmptyGURL());
+  navigation_manager()->CommitPendingItem(std::move(item));
+}
+
 // Tests NavigationManagerImpl::CommitPendingItem() with a valid pending item.
 TEST_P(NavigationManagerTest, CommitNonNilPendingItem) {
   // Create navigation manager with a single forward item and no back items.

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -335,10 +335,12 @@ void WKBasedNavigationManagerImpl::CommitPendingItem(
       // URL is correctly updated. This is a bug in WKWebView. Check to see if
       // the next or previous item matches, and update that item instead. If
       // nothing matches, still update the the currentItem.
-      if (item_url == net::GURLWithNSURL(back_forward_list.backItem.URL)) {
+      if (back_forward_list.backItem &&
+          item_url == net::GURLWithNSURL(back_forward_list.backItem.URL)) {
         SetNavigationItemInWKItem(back_forward_list.backItem, std::move(item));
-      } else if (item_url ==
-                 net::GURLWithNSURL(back_forward_list.forwardItem.URL)) {
+      } else if (back_forward_list.forwardItem &&
+                 item_url ==
+                     net::GURLWithNSURL(back_forward_list.forwardItem.URL)) {
         SetNavigationItemInWKItem(back_forward_list.forwardItem,
                                   std::move(item));
       } else {