allow login_block without username

Author: Elias Nygren

upcloud_api/server.py

@@ -18,14 +18,18 @@ def login_user_block(username, ssh_keys, create_password=True):
     Helper function for creating Server.login_user blocks.
     (see: https://www.upcloud.com/api/8-servers/#create-server)
     """
-    return {
-        'username': username,
+    block = {
         'create_password': 'yes' if create_password is True else 'no',
         'ssh_keys': {
             'ssh_key': ssh_keys
         }
     }
 
+    if username:
+        block['username'] = username
+
+    return block
+
 
 class Server(BaseAPI):
   """