models: cards: an empty string in members or label deletes the list
models: cards: an empty string in members or label deletes the list

There is currently no way to remove all members or all labels attached
to a card. If an empty string is provided, we can consider as a hint to
remove the list from the card.

GhassenRjab/wekan,wekan/wekan,libreboard/libreboard,GhassenRjab/wekan,wekan/wekan,wekan/wekan,wekan/wekan,wekan/wekan,GhassenRjab/wekan,libreboard/libreboard