GetAliasResponse.Values can be null

[gpetrou]

Elastic.Clients.Elasticsearch version: 8.19.0-preview.2

Elasticsearch version: 8.18.1

.NET runtime version: 8

Operating system version:

Description of the problem including expected versus actual behavior:
When ElasticsearchClient.Indices.GetAliasAsync fails because the alias does not exist, GetAliasResponse.Values is set to null. Either make it nullable or set an empty dictionary to it?

Steps to reproduce:

Expected behavior

Provide ConnectionSettings (if relevant):

Provide DebugInformation (if relevant):