Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-09-13' into staging

QAPI patches patches for 2021-09-13

# gpg: Signature made Mon 13 Sep 2021 08:53:42 BST
# gpg:                using RSA key 354BC8B3D7EB2A6B68674E5F3870B400EB918653
# gpg:                issuer "armbru@redhat.com"
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-qapi-2021-09-13:
  qapi: Fix bogus error for 'if': { 'not': '' }
  tests/qapi-schema: Cover 'not' condition with empty argument
  qapi: Bury some unused code in class Indentation
  qapi: Drop Indentation.__bool__()
  qapi: Fix a botched type annotation

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>