You can cast Pale Rider of Trostad even if you have no other cards in your hand. If you have no cards in hand as the last ability resolves, nothing happens.
Skulk matters only as blockers are chosen. Modifying either creatureâ€™s power after blockers are chosen wonâ€™t cause the attacking creature to become unblocked.
If you cause a creature to have 0 power or less, use the actual value (which may be negative) to determine whether it can block or be blocked. A creature with skulk and 0 or less power most likely wonâ€™t be blocked, but it wonâ€™t deal combat damage and wonâ€™t trigger any abilities that trigger when combat damage is dealt.