Lately we started to see failures on serverless:https://evergreen.mongodb.com/task_log_raw/dot_net_driver_serverless_tests_ubuntu__os~ubuntu_1804_auth~auth_ssl~ssl_compressor~zlib_test_serverless_netstandard21_patch_32a24dd2bddd528d2fe29668ea2d9d5107357c31_63251d260305b93a9ba48587_22_09_17_01_04_57/1?type=T#L681. This error happens because srv dns resolving might not be finished on time when we've started a test which leads to the situation when there is no servers in clusterDescription. However, no need to validate healthy servers for LB mode, so we can just skip server validation for LB cluster