Handle case where netbiosalias is omitted from AD payload

When netbiosalias is set in SMB configuration and omitted from the
payload in the AD form, it gets populated as an empty list causing
a ValidationError to be raised (because we don't allow changing
netbios configuration while AD is enabled). Unfortunately, some
howto guides online have recommended users to set the netbiosname
netbios alias to the same value, and as is the way of such things
this advice is both unnecessary and detremental. This means that users
who followed such guides are now unable to modify their AD configuration
through the webui (which no longer sets the netbiosalias by default).

Set the default for netbiosalias to None so that it does not default
to an empty list as a special value indicating that the existing
netbios aliases should be preserved.

src/middlewared/middlewared/plugins/activedirectory.py

@@ -71,7 +71,7 @@ class Config:
         Str('createcomputer'),
         NetbiosName('netbiosname'),
         NetbiosName('netbiosname_b'),
-        List('netbiosalias', items=[NetbiosName('alias')]),
+        List('netbiosalias', items=[NetbiosName('alias')], default=None),
         Bool('enable'),
         register=True
     )
@@ -122,6 +122,11 @@ async def nss_info_choices(self):
     @private
     async def update_netbios_data(self, old, new):
         must_update = False
+
+        # None here as opposed to empty list indicates to preserve current value
+        if new['netbiosalias'] is None:
+            new['netbiosalias'] = old['netbiosalias']
+
         for key in ['netbiosname', 'netbiosalias']:
             # netbios names are case-insensitive
             if key in new and old[key] != new[key]: