Fix: failed to create snapshot with DriverFilter
Fix: failed to create snapshot with DriverFilter

Previously, while creating snapshots, the code didn't pass through
scheduler therefore available capacity for backend wasn't checked.
This was supported by [1] which passed a RequestSpec object to
scheduler with snapshot and volume properties (the same RequestSpec
object is used for volumes).
But when the scheduler tries to get volume_type from RequestSpec obj,
unfortunately, it has a property named 'volume_type' but is None for
snapshots which breaks the scheduler when getting extra_specs from
the volume type (as the default value {} would've been set if the
property didn't exist).

Similar issue occurred when we supported untyped volumes and was
fixed by [2] and [3].

This patch sets the volume_type to an empty dict when it is found
None (which is the case while creating snapshots).

[1] https://review.opendev.org/#/c/509011/
[2] https://review.opendev.org/#/c/457431/
[3] https://review.opendev.org/#/c/471672/

Change-Id: I89a8cc42ca8984ee837a2b88f60ad126783282b9
Closes-Bug: #1856126

openstack/cinder,openstack/cinder,mahak/cinder,mahak/cinder