Rust: a zero-byte store file is rewritten as an empty v1 store

Soul's mutant deleting the empty-file arm of write_all_unlocked survived: no
test rewrote a zero-byte file. Without the arm the empty file reaches
store_header and is refused as truncated. The test pushes onto a zero-byte
file and expects a v1 store holding the pushed record.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/cultcache-rs/src/lib.rs

@@ -4876,6 +4876,19 @@ mod tests {
         Ok(())
     }
 
+    // A zero-byte file is an empty store with no header, not a truncated one: a rewrite writes it v1.
+    #[test]
+    fn push_all_rewrites_a_zero_byte_file_as_v1() -> Result<()> {
+        let temp = tempfile::tempdir()?;
+        let path = temp.path().join("store.msgpack");
+        std::fs::write(&path, [])?;
+        let mut store = SingleFileMessagePackBackingStore::new(&path);
+        store.push_all(&[snapshot_envelope("x", b"one")], PushAllOptions::default())?;
+        assert_eq!(store_header(&std::fs::read(&path)?, true)?.as_deref(), Some(STORE_FORMAT_V1));
+        assert_eq!(store.pull_all()?.len(), 1);
+        Ok(())
+    }
+
     #[test]
     fn a_rewrite_of_a_v1_store_stays_v1() -> Result<()> {
         assert_eq!(