The signature list counts a field as signed only when its /V resolves
to a dictionary, but signing an existing field refused any field that
had a /V key. A field with `/V null`, or a /V pointing at a null object,
was listed as unsigned and then refused with "the field is already
signed".
Both now ask the field for its signature dictionary through one
predicate. A field with a signature is still refused, and a field
without /V still signs as before.
Adds a_field_with_a_null_value_is_unsigned_and_can_be_signed, which
fails before this change.
Marco Arnulfo
f97f24cfbc