fix: client.Lag doesn't calculate lag for all consumergroups with client.Lag(ctx)

[0xgirish]

Getting lag as nil, in case Lag method was called as client.Lag(ctx), other methods of this library defaults to listing offsets and describing to all the groups/topics incase no group/topic is specified, Assuming the same behaviour for this method.

e.g.
Client.DescribeGroups

DescribeGroups describes either all groups specified, or all groups in the cluster if none are specified.

Client.ListTopics

ListTopics issues a metadata request and returns TopicDetails. Specific topics to describe can be passed as additional arguments. If no topics are specified, all topics are requested. Internal topics are not returned unless specifically requested. To see all topics including internal topics, use ListTopicsWithInternal.

[0xgirish]

After changes:

[0xgirish]

@twmb please review.

[0xgirish]

@twmb I have addressed the review comment.