Cannot do binary increment/decrement without initial value

Relevant failing FIT tests:

• BinaryTest.decrementThrowsDocumentNotFoundWhenInitialValueNotSupplied
• BinaryTest.incrementThrowsDocumentNotFoundWhenInitialValueNotSupplied

If the initial value is not set in the options, we set it to 0. It doesn't appear to be possible for the user to provide an input that gets mapped to unset in the core request's initial_value field (which is std::optional).