Fix logic in retype check for encryption changes

The check was looking for not None to decide if it can
call the driver.retype(..). Turns out the diff has an
empty dictionary when it didn't find any encryption diff.

We now just look at the converted boolean value of whatever
was in there so the empty dict is interpreted the right way.

Change-Id: Ic1ae3c6596f2a390b8f8a41495d1a58ee58eacdf
Closes-Bug: #1622762

cinder/tests/unit/test_volume.py

@@ -5302,7 +5302,11 @@ class VolumeMigrationTestCase(BaseVolumeTestCase):
                 mock.patch.object(db.sqlalchemy.api, 'volume_get') as mock_get:
             mock_get.return_value = volume
             _retype.return_value = driver
-            returned_diff = {}
+            returned_diff = {
+                'encryption': {},
+                'qos_specs': {},
+                'extra_specs': {},
+            }
             if encryption_changed:
                 returned_diff = {'encryption': 'fake'}
             _diff.return_value = (returned_diff, diff_equal)

cinder/volume/manager.py

@@ -2287,7 +2287,7 @@ class VolumeManager(manager.SchedulerDependentManager):
         # We assume that those that support pools do this internally
         # so we strip off the pools designation
         if (not retyped and
-                diff.get('encryption') is None and
+                not diff.get('encryption') and
                 vol_utils.hosts_are_equivalent(self.driver.host,
                                                host['host'])):
             try: