libtracker-data: Join SERVICE with triple patterns in arbitrary order

There was one case left to fix in #248 (closed):

  SELECT ?urn {
      {
          SELECT ?urn {
              ?urn a nfo:DataContainer .
          }
      }
      UNION {
          SELECT ?urn {
              SERVICE <dbus:org.freedesktop.Tracker3.Miner.Files> {
                  SELECT ?urn  {
                      ?urn a nmm:Photo .
                  }
              }
              ?urn nie:isPartOf <urn:photo-album:test>
          }
      }
  }

Might result in the second SELECT matching ?urn as a string on the SERVICE side, and as a resource ID from the nie:isPartOf part.

We already handle joining with triple patterns the other way around (commit 14392fdb), so make sure we handle SERVICE graph patterns in that order within commutative operations.