Works without aliases shouldn't be included in user,alias index

[petli]

The intention in the data model is that Work.alias should be optional. However, when creating works without an alias it is still included in this index, despite the sparse flag.

Work.index({ 'owner.user': 1, 'alias': 1 }, { unique: true, sparse: true });

Which results in a duplicate key error when the same users creates another work without an alias.

[ebalder]

It's a problem with the compound sparse key implementation from MongoDB. If any of both keys is present, both are going to be included in the index.

Directly in the mongo shell:
- adding a work with no owner nor alias: no problem
- adding a work with duplicate owner and no alias: dup-key error
- adding a work with duplicate alias and no owner: dup-key error

related: https://jira.mongodb.org/browse/SERVER-2193

I would then proppose to add the work id in place of the alias if it's not set. But that would increase the size of the index.