RandomString don't skip superclass validation

Ensure the superclass validation happens as well as the class-local
additional stuff.

Change-Id: I3fdc6e5d6a4ca741cdd12ca0ed6ceb908c03ae9f
Closes-Bug: #1392754
(cherry picked from commit 02c4b2441e12ed12fa58ddd9858b5d91314668ee)
Merge "RandomString don't skip superclass validation" into stable/juno