An empty AttrValue::String (or a StringArray of empty strings) was
written with a size-0 fixed-length string type. libhdf5 rejects that
("invalid datatype size"), and the failure takes every attribute on the
object with it. Strings are now at least 1 byte, NUL-padded, which is
how h5py stores "" and reads back as "" in both h5py and our reader.
check_encodable also refuses a size-0 string type passed in directly.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>