[YAMLTraits] Fix std::optional input on empty documents (#68947)

When the input document is non-empty, `mapOptional` works as expected,
setting `std::optional` to `std::nullopt` when the field is not present.
When the input document is empty, we hit a special case inside of
`Input::preflightKey` that results in `UseDefault = false`, which
results in the `std::optional` erroneously being set to a non-nullopt
value. `preflightKey` is changed to set `UseDefault = true` in this case
to make the behavior consistent between empty and non-empty documents.