Prevent a Mongo error when tag_ids are set to nil

At present, when the tag ids for a type of tag are set to nil (eg.
'organisation_ids = nil'), the operation will raise an error as
the Mongo query is expecting an array.

This changes the writer method to make sure we only request tag IDs
as an array when we fetch tags from the database.
Merge pull request #159 from alphagov/support-nil-tag-type-ids

Prevent a Mongo error when tag_ids are set to nil