Limited liability companies cannot have a value less than zero in general. However, WeWork’s lawsuit against SoftBank could very well be successful; effectively making it negative for SoftBank.
WeWork's lawsuit against softbank only succeeds if it lives long enough to get to a court decision, and Softbank are in quite a strong position to stop that happening.
When the final lifeline for your company is suing your primary investor over competing definitions of just how terrible the company is performing then I’m not sure it’s worth much for either party!