Handle omitted optional HDF5 strings in Python readers

[paulromano]

Description

This PR is an alternative to #4088 that preserves the existing HDF5 convention of omitting empty string attributes and datasets. Rather than changing the low-level string writer and every file it produces, this PR makes readers that consume optional strings supply the appropriate default when a field is absent. In particular:

• StatePoint.path now returns an empty string when the statepoint has no path attribute. This fixes the MWE from Write empty strings to hdf5 #4088, where Model.run() does not pass an input path and the C++ writer consequently omits the empty attribute.
• UnstructuredMesh.from_hdf5() now accepts a missing filename dataset. A filename is unavailable when an unstructured mesh was constructed from an in-memory mesh rather than a file, which also resolves LibMesh::filename_ attribute not written when pointer-based constructors are used. #2285.

Closes #2285

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[GuySten]

I fixed another unguarded read.