migration: use "" instead of (null) for tls-authz

migration: use "" instead of (null) for tls-authz

run:
(qemu) info migrate_parameters
announce-initial: 50 ms
...
announce-max: 550 ms
multifd-compression: none
xbzrle-cache-size: 4194304
max-postcopy-bandwidth: 0
 tls-authz: '(null)'

Migration parameter 'tls-authz' is used to provide the QOM ID
of a QAuthZ subclass instance that provides the access control
check, default is NULL. But the empty string is not a valid
object ID, so use "" instead of the default. Although it will
fail when lookup an object with ID "", it is harmless, just
consistent with tls_creds.

As a bonus, this patch also fixed the bad indentation on the
last line and removed 'has_tls_authz' redundant check in
'hmp_info_migrate_parameters'.

Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Message-Id: <119f539a9f4d198bc3bcced46b8280520d60bc51.1585100802.git.maozhongyi@cmss.chinamobile.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>