ยง8.4 Unavailable / sentinel value (post-decode, pre-scale raw pattern).
optional, e.g. "not_implemented", "over_range"
from field: string meaning = 2;
from field: int64 raw = 1;
from message moddef.v1.NaValue
ยง8.4 Unavailable / sentinel value (post-decode, pre-scale raw pattern).