Added return statement to retype_volume in v2 volumes_client

This commit adds a missing return statement to the retype_volume API
endpoint in the v2 volumes_client. Otherwise, None, instead of an
empty dict, is returned for the response body.

Closes-Bug: #1703997
Partially Implements: blueprint tempest-lib-missing-test-coverage
Change-Id: I1a308f486fd7b14a9111a3433284e3c4abf65bd3
3 files changed