Write empty strings to hdf5

[GuySten]

Description

Currently we do not write empty strings to hdf5 which makes alot of fields conditionaly there.
This is a source for errors like the MWE below.

This PR change that behavior so that the fields are guaranteed to exist and can be empty.

MWE

import openmc
# --- a minimal real model: 10 cm water sphere, fixed source at the center ---
water = openmc.Material()
water.add_nuclide('H1', 2.0)
water.add_nuclide('O16', 1.0)
water.set_density('g/cm3', 1.0)
sphere = openmc.Sphere(r=10.0, boundary_type='vacuum')
cell = openmc.Cell(fill=water, region=-sphere)
settings = openmc.Settings()
settings.run_mode = 'fixed source'
settings.particles = 100
settings.batches = 5
settings.source = openmc.IndependentSource(space=openmc.stats.Point())
model = openmc.Model(
geometry=openmc.Geometry([cell]),
materials=openmc.Materials([water]),
settings=settings,
)
# Model.run() forwards path_input=None (model.py:963), so the executable is
# invoked without -i and settings::path_input stays empty.
sp_file = model.run()
sp = openmc.StatePoint(sp_file, autolink=False)
print('statepoint:', sp_file)
print('version: ', sp.version)
print('attrs: ', sorted(sp._f.attrs)) # 'path' is missing
print(sp.path) # KeyError: 'path'

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)