Fix VeraCrypt volumes not being automatically opened in Nautilus when they should

Opening a volume via Nautilus’ “Open With Unlock VeraCrypt Volumes” should automatically open the volume in Nautilus after unlocking it, but this fails with:

INFO:unlock_veracrypt_volumes.volume:Unlocking volume /dev/loop1
INFO:unlock_veracrypt_volumes.volume:Opening volume /dev/dm-1
Traceback (most recent call last):
  File "/usr/local/lib/python3/dist-packages/unlock_veracrypt_volumes/volume.py", line 227, in mount_cb
    self.open()
  File "/usr/local/lib/python3/dist-packages/unlock_veracrypt_volumes/volume.py", line 273, in open
    mount_points = self.udisks_object.get_filesystem().props.mount_points
AttributeError: 'NoneType' object has no attribute 'props'

Feature Branch: segfault:bugfix/15954-veracrypt-volumes-not-opened

Parent Task: #14468

Original created by @segfault on 15954 (Redmine)

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