fix(format): give empty string attributes a 1-byte type

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]>

crates/clawhdf5-format/src/datatype.rs

@@ -842,6 +842,9 @@ impl Datatype {
                     opaque_tag_text(tag).len()
                 )))
             }
+            Datatype::String { size: 0, .. } => Err(FormatError::SerializationError(
+                "fixed-length string datatype of size 0 (libhdf5 requires at least 1 byte)".into(),
+            )),
             Datatype::Compound { members, .. } => members
                 .iter()
                 .try_for_each(|m| m.datatype.check_encodable()),

crates/clawhdf5-format/src/type_builders.rs

@@ -384,7 +384,11 @@ pub(crate) fn build_attr_message(name: &str, value: &AttrValue) -> AttributeMess
             raw_data: data.clone(),
         },
         AttrValue::String(s) => {
-            let bytes = s.as_bytes();
+            // A fixed-length string type must be at least 1 byte: libhdf5
+            // rejects size 0 ("invalid datatype size") and with it every
+            // attribute on the object. h5py stores "" as one NUL byte.
+            let mut bytes = s.as_bytes().to_vec();
+            bytes.resize(bytes.len().max(1), 0);
             AttributeMessage {
                 name: name.to_string(),
                 datatype: Datatype::String {
@@ -393,11 +397,12 @@ pub(crate) fn build_attr_message(name: &str, value: &AttrValue) -> AttributeMess
                     charset: CharacterSet::Utf8,
                 },
                 dataspace: scalar_ds(),
-                raw_data: bytes.to_vec(),
+                raw_data: bytes,
             }
         }
         AttrValue::StringArray(arr) => {
-            let max_len = arr.iter().map(|s| s.len()).max().unwrap_or(0);
+            // At least 1 byte per element, as for a single string.
+            let max_len = arr.iter().map(|s| s.len()).max().unwrap_or(0).max(1);
             let mut raw = Vec::new();
             for s in arr {
                 let mut b = s.as_bytes().to_vec();