Can request CollectionViewModel itemAtIndexPath with nil

ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/collection_view/collection_view_model.h"
 
 #include "base/mac/foundation_util.h"
 #include "base/mac/scoped_nsobject.h"
 #import "ios/chrome/browser/ui/collection_view/cells/collection_view_item.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 58 matching lines @@
 
   [model addSectionWithIdentifier:SectionIdentifierCheese];
   [model addItemWithType:ItemTypeCheeseCheddar
       toSectionWithIdentifier:SectionIdentifierCheese];
   [model addItemWithType:ItemTypeCheeseGouda
       toSectionWithIdentifier:SectionIdentifierCheese];
   [model addItemWithType:ItemTypeCheesePepperJack
       toSectionWithIdentifier:SectionIdentifierCheese];
 
   // Check there are some items but not more.
+  EXPECT_EQ(NO, [model hasItemAtIndexPath:nil]);
   EXPECT_EQ(YES, [model hasItemAtIndexPath:[NSIndexPath indexPathForItem:0
                                                                inSection:0]]);
   EXPECT_EQ(YES, [model hasItemAtIndexPath:[NSIndexPath indexPathForItem:2
                                                                inSection:0]]);
   EXPECT_EQ(NO, [model hasItemAtIndexPath:[NSIndexPath indexPathForItem:3
                                                               inSection:0]]);
   EXPECT_EQ(NO, [model hasItemAtIndexPath:[NSIndexPath indexPathForItem:0
                                                               inSection:1]]);
 
   // Check the collection view data sourcing methods.
@@ skipping 494 matching lines @@
 
   EXPECT_FALSE([model footerForSectionWithIdentifier:SectionIdentifierCheese]);
   EXPECT_FALSE([model footerForSection:cheeseSection]);
 
   EXPECT_EQ(weasleyFooter.get(),
             [model footerForSectionWithIdentifier:SectionIdentifierWeasley]);
   EXPECT_EQ(weasleyFooter.get(), [model footerForSection:weasleySection]);
 }
 
 }  // namespace