Fix SetUUIDMetadata when no profile, email or name is set

[luna-duclos]

The REST API for SetUUIDMetadata allows those fields to be unset or null but validates that if they are set they are valid.
The Go SDK always sets those fields, resulting in calls such as this one to fail:

s.pubnubClient.SetUUIDMetadata().UUID(userInfo.EmbarkUser.ExternalID.String()).Custom(pubnubUserInfoMap).Execute()

This should work, and the way to make this work is to add the omitEmpty option

[kleewho]

@luna-duclos Thanks for the PR. We'll look into it