Drop attr defaultServerList if removing the last server

This otherwise returns a syntax error if trying to set
an empty value.

Related: https://pagure.io/freeipa/issue/6760

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipaserver/plugins/server.py

@@ -615,8 +615,10 @@ class server_del(LDAPDelete):
             srvlist = srvlist.split()
             if master in srvlist:
                 srvlist.remove(master)
-                attr = ' '.join(srvlist)
-                ret['defaultServerList'] = attr
+                if not srvlist:
+                    del ret['defaultServerList']
+                else:
+                    ret['defaultServerList'] = ' '.join(srvlist)
                 conn.update_entry(ret)
         except (errors.NotFound, errors.MidairCollision,
                 errors.EmptyModlist):