dbcore: types translate null values on assignment

In preparation for #660, where we will allow MediaFile to expose None values
when tags are missing (and consume None to remove tags). This makes it
possible to hide nullness in the rest of beets by translating None to a
suitable zero-ish value on field assignment.

Types can of course opt out of this to preserve a distinct null value. We do
this now for the album_id field, which needs to be null to indicate
singletons.

Type.normalize() also enables more sophisticated translations (e.g., an
integer field could round off float values assigned into it) in the future.