Handle both null in Document.equals

Author: adwsingh

core/src/main/java/software/amazon/smithy/java/core/serde/document/Document.java

@@ -683,7 +683,9 @@ static boolean equals(Object left, Object right) {
      * @return true if they are equal.
      */
     static boolean equals(Object left, Object right, int options) {
-        if (left instanceof Document l && right instanceof Document r) {
+        if (left == null && right == null) {
+            return true;
+        } else if (left instanceof Document l && right instanceof Document r) {
             if (l == r) {
                 return true;
             }