base-item, utils: Avoid CRITICALs when reading an item's metadata

If the Tracker extractor has not yet run for an item then various
variables in the query will be unbound. This would led to things like:
  Tracker-CRITICAL **: tracker_sparql_cursor_real_get_integer:
    assertion 'tracker_sparql_cursor_get_value_type (self, column) ==
    TRACKER_SPARQL_VALUE_TYPE_INTEGER' failed
1 job for wip/rishi/fix-tracker-assertions in 1 minute and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
canceled #895185
flatpak
flatpak

00:01:04