Tag attribute type needs to be initialized to a non-empty string

As a part of tagsync updates for handling Atlas V2 APIs, a regression was introduced which set tag attribute type to an empty string when creating service-tags structure from a AtlasEntityNotification object. When saving attribute-definition to Oracle DB, this empty string tag was treated as a null, which caused the database insert to fail (type column is defined as non-null field in schema for x_tag_attr_def table).

As tag attribute type values are uninterpreted in Ranger, it is safe to set attribute type to 'string' instead of to an empty string.