fix(dynamodb2): Fix update_item nested insert (#3355)

When comparing old and new values when doing a nested item update, the
`!=` implementation fails when the value being compared is `None`. This
results in an exception when trying to insert a new item into a nested
map. So just do a quick check that the original value is exists before
doing the comparison, as the `None` default is what is tripping this.

moto/dynamodb2/responses.py

@@ -826,7 +826,7 @@ class DynamoHandler(BaseResponse):
                         original.get(key, None), changed[key]
                     )
                     for key in changed.keys()
-                    if changed[key] != original.get(key, None)
+                    if key not in original or changed[key] != original[key]
                 }
             elif type(changed) in (set, list):
                 if len(changed) != len(original):

tests/test_dynamodb2/test_dynamodb.py

@@ -3936,6 +3936,37 @@ def test_update_supports_list_append_maps():
     )
 
 
+@requires_boto_gte("2.9")
+@mock_dynamodb2
+def test_update_supports_nested_update_if_nested_value_not_exists():
+    dynamodb = boto3.resource("dynamodb", region_name="us-east-1")
+    name = "TestTable"
+
+    dynamodb.create_table(
+        TableName=name,
+        KeySchema=[{"AttributeName": "user_id", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "user_id", "AttributeType": "S"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 5, "WriteCapacityUnits": 5},
+    )
+
+    table = dynamodb.Table(name)
+    table.put_item(
+        Item={"user_id": "1234", "friends": {"5678": {"name": "friend_5678"}},},
+    )
+    table.update_item(
+        Key={"user_id": "1234"},
+        ExpressionAttributeNames={"#friends": "friends", "#friendid": "0000",},
+        ExpressionAttributeValues={":friend": {"name": "friend_0000"},},
+        UpdateExpression="SET #friends.#friendid = :friend",
+        ReturnValues="UPDATED_NEW",
+    )
+    item = table.get_item(Key={"user_id": "1234"})["Item"]
+    assert item == {
+        "user_id": "1234",
+        "friends": {"5678": {"name": "friend_5678"}, "0000": {"name": "friend_0000"},},
+    }
+
+
 @mock_dynamodb2
 def test_update_supports_list_append_with_nested_if_not_exists_operation():
     dynamo = boto3.resource("dynamodb", region_name="us-west-1")