Fix input_path resolving empty filenames to cwd

[Arthur031221]

Description

Anyone who reads back an in-memory UnstructuredMesh or DAGMCUniverse
(one built from a MOAB/libmesh instance in memory and never backed by a
source file) from a summary or statepoint HDF5 file gets a filename
attribute that silently points at the current working directory instead
of indicating that there is no file.

UnstructuredMesh.from_hdf5 and DAGMCUniverse.from_hdf5 pass an empty
string for filename when the HDF5 group has no filename dataset
(added in #4118 to handle meshes/universes with no source file). That
empty string reaches openmc.utility_funcs.input_path, which calls
Path(filename).resolve() whenever openmc.config['resolve_paths'] is
True (the default). Path('').resolve() resolves to the process's
current working directory, not to an empty path, so mesh.filename or
dagmc_universe.filename ends up set to whatever directory happened to
be current when the file was read. Code that checks mesh.filename (for
example mesh.filename.exists() to decide whether a source file is
available) gets a wrong answer, since the working directory always
exists.

This also means the two round-trip assertions added in #4118
(tests/unit_tests/test_mesh.py::test_umesh_from_hdf5_without_filename
and
tests/unit_tests/test_universe.py::test_dagmc_universe_from_hdf5_without_filename,
both asserting filename == Path()) do not actually hold under the
default configuration.

input_path now returns an empty, unresolved Path when given an empty
filename, before the resolve_paths branch runs. No other caller of
input_path (cross section paths, source library/path, settings files)
passes an empty string in normal use, so this only changes behavior for
the "no file" case.

Fixes # (none filed; found by code review)

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)