ASDataController: Correctly Handle Nil IndexPath in nodeForIndexPath: Methods (#2856)

* ASDataController: Correctly handle fetching node at nil index path (return nil).

* Be more aggressive, in order to avoid needlessly waiting for updates to complete

Author: Adlai Holler

AsyncDisplayKit/ASCollectionView.mm

@@ -612,6 +612,10 @@ - (ASCellNode *)nodeForItemAtIndexPath:(NSIndexPath *)indexPath
 
 - (NSIndexPath *)convertIndexPathFromCollectionNode:(NSIndexPath *)indexPath waitingIfNeeded:(BOOL)wait
 {
+  if (indexPath == nil) {
+    return nil;
+  }
+  
   // If this is a section index path, we don't currently have a method
   // to do a mapping.
   if (indexPath.item == NSNotFound) {

AsyncDisplayKit/ASTableView.mm

@@ -591,6 +591,10 @@ - (nullable NSIndexPath *)validateIndexPath:(nullable NSIndexPath *)indexPath
 
 - (nullable NSIndexPath *)indexPathForNode:(ASCellNode *)cellNode waitingIfNeeded:(BOOL)wait
 {
+  if (cellNode == nil) {
+    return nil;
+  }
+
   NSIndexPath *indexPath = [_dataController completedIndexPathForNode:cellNode];
   indexPath = [self validateIndexPath:indexPath];
   if (indexPath == nil && wait) {

AsyncDisplayKit/Details/ASDataController.mm

@@ -952,6 +952,9 @@ - (NSUInteger)completedNumberOfRowsInSection:(NSUInteger)section
 - (ASCellNode *)nodeAtIndexPath:(NSIndexPath *)indexPath
 {
   ASDisplayNodeAssertMainThread();
+  if (indexPath == nil) {
+    return nil;
+  }
 
   NSArray *contexts = _nodeContexts[ASDataControllerRowNodeKind];
   NSInteger section = indexPath.section;
@@ -971,6 +974,9 @@ - (ASCellNode *)nodeAtIndexPath:(NSIndexPath *)indexPath
 - (ASCellNode *)nodeAtCompletedIndexPath:(NSIndexPath *)indexPath
 {
   ASDisplayNodeAssertMainThread();
+  if (indexPath == nil) {
+    return nil;
+  }
 
   NSArray *completedNodes = [self completedNodes];
   NSInteger section = indexPath.section;