Torts — Negligence — Workers’ compensation immunity — Estoppel — Negligence action filed against employer after plaintiff voluntarily dismissed workers’ compensation petition — Employer is not estopped from asserting workers’ compensation immunity where, in compensation proceeding, employer asserted that no work accident causing injury occurred at all, not that accident occurred outside the scope of employment — An employer is not estopped from asserting workers’ compensation exclusivity merely because it had denied compensability of an alleged workplace injury — Employer is entitled to litigate whether a compensable accident occurred in a compensation forum