Make Equality.hash accept null as an argument.

This just makes it easier to use the equality directly on a nullable type.
The `isValidKey` check still say no for `null`.

Review-Url: .
diff --git a/lib/src/equality.dart b/lib/src/equality.dart
index a970664..2c6c272 100644
--- a/lib/src/equality.dart
+++ b/lib/src/equality.dart
@@ -164,7 +164,7 @@
   int hash(List<E> list) {
-    if (elements == null) return null.hashCode;
+    if (list == null) return null.hashCode;
     // Jenkins's one-at-a-time hash function.
     // This code is almost identical to the one in IterableEquality, except
     // that it uses indexing instead of iterating to get the elements.