libtracker-miner: Fix possible warnings

This might be called in the right circumstances that the current_dir
is NULL, cater for that. Fixes possible warnings like:

tracker-miner-f[2965]: g_file_equal: assertion 'G_IS_FILE (file1)' failed
tracker-miner-f[2965]: g_file_has_prefix: assertion 'G_IS_FILE (file)' failed

(cherry-picked from commit 5a052219)

Closes: https://gitlab.gnome.org/GNOME/tracker/-/issues/342