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
(cherry picked from commit 1e67072122)

cinder/scheduler/filter_scheduler.py

@@ -287,7 +287,12 @@ class FilterScheduler(driver.Scheduler):
         # takes 'resource_XX' and 'volume_XX' as input respectively, copying
         # 'volume_XX' to 'resource_XX' will make both filters happy.
         volume_type = request_spec.get("volume_type")
-        resource_type = volume_type if volume_type is not None else {}
+        # When creating snapshots, the value of volume_type is None here
+        # which causes issues in filters (Eg: Bug #1856126).
+        # To prevent that, we set it as an empty dictionary here.
+        if volume_type is None:
+            volume_type = {}
+        resource_type = volume_type
 
         config_options = self._get_configuration_options()