The mtime is stored as nfo:fileLastModified or nie:contentLastModified
in the Tracker database. These properties are of type xsd:dateTime,
and are returned as strings, not booleans. Hence it led to:
  Tracker-CRITICAL **: tracker_sparql_cursor_real_get_boolean:
    assertion '_tmp0_ == TRACKER_SPARQL_VALUE_TYPE_BOOLEAN' failed

These CRITICALs were more often (always?) triggered when using
TrackerSparqlConnection's 'bus' backend that uses D-Bus to access the
Tracker database. eg., the Flatpak builds that don't bundle the Tracker
D-Bus services.

Fallout from 3ad413e3

!149