Improve Date Element to better handle zero-sized Dates

[FreezyLemon]

The spec allows zero-sized Date Elements, which are equivalent to a value of 0.

Some improvements were discussed in #128 to allow representing "empty" Date Elements with a zero-sized type on the Rust side.

Another small improvement would be on the serialization side, where the Date is currently always serialized as a uinteger. A Date of "0" could also be represented by an empty Element to save some bytes.