The Law of Ukraine "On the Legal Regime of Martial Law" and the Resolution of the Cabinet of Ukraine on Restrictions of Freedom of Movement of Citizens in Areas Where Martial Law Is Introduced do not say anything about spending the night in a tent.
But in practice, it is better to spend the night in a tent coordinate with local authorities. After all, violation of curfew rules can lead to a fine, community service or even administrative arrest (up to 15 days).