Fix HDF5 dataset creation failure when nbodies == 0

[Copilot]

When a node holds zero particles, the chunk clamping logic produces chunk >= 1 against a zero-length dataspace (dims[0] == 0), which violates HDF5's constraint that chunk dimensions must not exceed dataset extent — causing dataset creation to fail. Additionally, std::clamp<int>(0, 1, 0) (lo > hi) is undefined behaviour.

Changes

• write_H5 (H5:: path, ~line 2645): Guard the chunk/compression property block with h5_particles.size() > 0; an empty dataset is created without chunking or filters.
• write_HDF5 (HighFive path, ~line 2504): Same guard using nbodies > 0, covering all four dcpl property lists (1-D, 3-D, iattrib, dattrib).

// Before — UB + HDF5 error when nbodies == 0
if (H5compress or H5chunk) { ... std::clamp<int>(chunk, 1, nbodies) ... }

// After — skip filters entirely for empty datasets
if ((H5compress or H5chunk) and nbodies > 0) { ... }

---

⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.