Allow empty EIDs field

[bretg]

prebid/Prebid.js#9841 reports that PBS (at least Java) is choking on an empty user.eids:

request.user.eids must contain at least one element or be undefined

I don't recall why we would be validating this field -- we should discuss this and other empty-array locations.

[bretg]

PBS-Go also rejects an empty EIDs array. Besides that validation here are the others:

• source must be defined and unique
• must have at least one uids entry
• each uids entry must have an id

We're ok relaxing this rule. If a bid adapter has trouble with an empty user.eids, we will reach out to the bidder and work with them.

[SyntaxNode]

Implemented in PBS-Go 0.253
Released with PBS-Java 1.119