A hole in the MtG rules?

Suppose that the card Wild Evocation is on the battlefield.

At the beginning of each player's upkeep, that player reveals a card at random, and if it's a land card, puts it onto the battlefield, else the player casts it without paying its mana cost. This is not optional. The card must be cast if possible (that is, if it's a spell that for example targets something, if there is something legal that it can target, it must be cast targeting one of those things.)

The casting is so mandatory, in fact, that if casting has some mandatory additional costs, they must be paid if possible. The oracle rulings state:

If casting the revealed card involves paying a mandatory additional cost (such as the one Fling has), the player casting that card must pay that cost if able.

This isn't optional either.

Now, what happens if the revealed card is, for example, Disaster Radius?

The mandatory cost of revealing a creature card must be paid if the player has a creature card in hand. However, a player cannot be forced to reveal their hand to corroborate that they don't have such a card. (No effect in either card requires the entire hand to be revealed, and no rule allows other players to demand the entire hand to be revealed.)

So how is this situation handled? How to corroborate that a player is telling the truth if they claim that they have no creature cards in hand?

The answer is that the rules of the game simply don't cover this situation. In other words, the rules do not offer any standard mechanism for other players to corroborate the situation.

The only thing that the other player or players can do is to ask a judge to examine the player's hand, if in a tournament setting, or if in a casual setting either simply trust the other player or ask an impartial third party.

Not that this situation is at all common, but I find it interesting that there's something that the rules of the game just don't cover.

Update: A rule introduced with Magic 2014 will cover this precise situation. The new rule will be that the player will not be forced to reveal hidden information (and thus, if the player chooses not to, he or she doesn't cast the card.)