Fix crash if batch update blocks are nil although method declaration allows nullable (#2599)

AsyncDisplayKit/ASCollectionView.mm

@@ -653,7 +653,9 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   ASDisplayNodeAssertMainThread();
   
   [_dataController beginUpdates];
-  updates();
+  if (updates) {
+    updates();
+  }
   [_dataController endUpdatesAnimated:animated completion:completion];
 }
 

AsyncDisplayKit/ASTableNode.mm

@@ -521,7 +521,9 @@ ASEnvironmentCollectionTableSetEnvironmentState(_environmentStateLock)
 - (void)performBatchAnimated:(BOOL)animated updates:(void (^)())updates completion:(void (^)(BOOL))completion
 {
   [self.view beginUpdates];
-  updates();
+  if (updates) {
+    updates();
+  }
   [self.view endUpdatesAnimated:animated completion:completion];
 }
 

AsyncDisplayKitTests/ASCollectionViewTests.mm

@@ -798,4 +798,14 @@
   XCTAssertNotEqualObjects(oldHeader, newHeader);
 }
 
+- (void)testThatNilBatchUpdatesCanBeSubmitted
+{
+  __block ASCollectionViewTestController *testController = [[ASCollectionViewTestController alloc] initWithNibName:nil bundle:nil];
+  __block ASCollectionNode *cn = testController.collectionNode;
+  
+  // Passing nil blocks should not crash
+  [cn performBatchUpdates:nil completion:nil];
+  [cn performBatchAnimated:NO updates:nil completion:nil];
+}
+
 @end

AsyncDisplayKitTests/ASTableViewTests.m

@@ -705,6 +705,15 @@
   }
 }
 
+- (void)testThatNilBatchUpdatesCanBeSubmitted
+{
+  ASTableNode *node = [[ASTableNode alloc] initWithStyle:UITableViewStylePlain];
+  
+  // Passing nil blocks should not crash
+  [node performBatchUpdates:nil completion:nil];
+  [node performBatchAnimated:NO updates:nil completion:nil];
+}
+
 @end
 
 @implementation UITableView (Testing)