libtracker-data: Ensure blank node row ids are integer typed

Since commit 6cf31683 tracker sometimes returns an unqualified row id to applications that are asking for a fully qualfied blank node urn.

This is because sometimes it generates URN with a text type instead of an integer type, and so SparqlPrintIRI fails to recognize it as a blank node identifier.

The result is that gnome-photos fails to match up album collections internally between different queries, because sometimes tracker returns the fully qualified urn and sometimes it returns the bare row id.

This commit address the problem by introducing a cast in the query that is returning the row id as text instead of as an integer.

Fix suggested by Carlos Garnacho.

Closes #363 (closed)