Add utility for DiffableDataSources to create an empty snapshot for custom list construction

Sources/DiffableDataSource.BaseAdapter.swift

@@ -155,6 +155,17 @@ extension DiffableDataSource {
             )
         }
         
+        /**
+         Creates a new empty `ListSnapshot` suitable for building custom lists inside subclass implementations of `apply(_:animatingDifferences:completion:)`.
+         */
+        public func makeEmptySnapshot() -> ListSnapshot<O> {
+            
+            return .init(
+                diffableSnapshot: .init(),
+                context: self.dataStack.unsafeContext()
+            )
+        }
+
         /**
          Returns the number of sections