"We entirely endorse the view that merely not giving evidence cannot, of itself, be a factor tending to show the person is not to be believed. It is also, however, and equally clearly, not a factor tending to show that the person is to be believed. If doubts have been raised about the credibility or plausibility of certain evidence, and the facts related by that evidence are not supported by other evidence, the position may be that the fact finder remains in doubt. The consequence of a fact finders doubt is or may be that the burden of proof is not discharged and so the party who has the burden of proof loses his case."