nautils-search-engine-tracker: do not pass NULL to g_date_time_new_from_iso_8601

That function does not accept a NULL, fixing the critical:

GLib-CRITICAL **: 12:05:52.144: g_time_val_from_iso8601: assertion 'iso_date != NULL' failed

This is possible since tracker can return a NULL[1] "if the column is not in the [0,#ncolumns] range"

[1] https://gnome.pages.gitlab.gnome.org/tracker/docs/developer/tracker-sparql-cursor.html?gi-language=c#tracker_sparql_cursor_get_string