fix(thermo): guard empty legend under matplotlib 3.11+

[jmmshn]

Fixes the tests/test_thermo.py::test_plotter failure that has been red on main since 2026-06-12.

Root cause

matplotlib 3.11.x tightened _parse_legend_args. On the failing branch of test_plotter, both artists + handle and legends_txt + leg are empty; the prior axis.legend(handles=[], labels=[], ...) call previously warned but is now a ValueError.

Fix

Skip axis.legend(...) entirely when the combined handles/labels are empty. No behavior change for callers that actually pass artists — the old empty-legend was already a no-op modulo the warning.

Verification

pytest tests/test_thermo.py::test_plotter -x  # was ValueError, now passes
pytest tests/test_thermo.py -x                # full file green

Reproduced pre-fix locally on matplotlib==3.11.1.

Bead: pmg-defects-epic.7

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.78%. Comparing base (12c1f80) to head (a9a6141).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pymatgen/analysis/defects/thermo.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   90.94%   90.78%   -0.17%     
==========================================
  Files          14       14              
  Lines        2265     2268       +3     
  Branches      287      288       +1     
==========================================
- Hits         2060     2059       -1     
- Misses        118      120       +2     
- Partials       87       89       +2     

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