qapi/expr.py: Add assertion for union type 'check_dict'

qapi/expr.py: Add assertion for union type 'check_dict'

mypy isn't fond of allowing you to check for bool membership in a
collection of str elements. Guard this lookup for precisely when we were
given a name.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210421182032.3521476-6-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>