List fields don't return None if the tag doesn't exist

[jtpavlock]

If a list tag does not exist, it returns an empty list rather than None, but if a non-list tag does not exist, None is returned.

I believe non-existent list tags should also return None so that this behavior is consistent.

[sampsyo]

That's an interesting suggestion! Which tags are you thinking of that produce None when the field is missing? We actually try to avoid that in most cases—the idea is to simplify client code so it doesn't need to constantly check the type of every field. For example, I believe artist is '', not None, when the underlying metadata is missing.

This design decision is certainly a trade-off. The simpler client code comes at the cost of not being able to distinguish the existence of tags.

[jtpavlock]

Which tags are you thinking of that produce None when the field is missing?

Referencing #38 and #39, I thought this was the desired behavior if a tag is missing altogether.

[sampsyo]

Ah, you're totally right; I was thinking of the old behavior before we started making this change. (And also what we do at the beets layer, which is to coalesce missing values into default values.) In that case, this issue makes sense to me!