No, you do not have any grounds for a lawsuit. You could have refused to do this, and while that might have cost you your job, you therefore made a voluntary choice to keep your job while cleaning up after the body. The law does not provide compensation because you made a voluntary choice which you regret, especially since employers have the right to require their employees to do anything which is not intrinsically illegal or a crime--and cleaming up after  dead body is not intrinscially illegal.