FAST: Do not set previous value to empty if delta operator is applied

FAST Spec says that in case of delta operator for optional integers
and decimals the previous value is not set to empty but is left
untouched if the value is absent

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>