Invoice Ninja returns `null` for many nominally-string fields and omits
others, so one model that fails on either takes the whole response down with
it. Rather than trusting each field to carry the right serde attribute, this
serialises `Default`, then nulls and separately removes each field in turn,
and requires the result to still decode.
A wrong *type* is deliberately not covered: that is meant to be a hard
error, so it is not something to absorb.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
