Workers’ compensation — Pretrial stipulation — Amendments — Judge of Compensation Claims erred in precluding claimant’s amendment of the pretrial stipulation to include consideration of waiver argument under section 440.20(4) on the basis that it was improper under the Administrative Code rule 60Q-6.113(2)(a) and (6) where the parties agreed that the pretrial stipulation could be amended unilaterally up to thirty days prior to hearing