fix: treat empty-string solver_options_file as unset

[DLWoodruff]

Summary

• shared_options() and apply_solver_specs() gated the load_solver_options_file() call on _hasit(cfg, ...), which returns True for any non-None value — including \"\".
• Callers that default solver_options_file (or any {name}_solver_options_file) to \"\" as a "no file" sentinel were hitting FileNotFoundError: [Errno 2] No such file or directory: ''.
• Replace _hasit with a plain truthiness check on the path. Both None and \"\" are now treated as unset, matching conventional file-path-arg semantics. No other _hasit semantics changed.

Test plan

• Two regression tests added in test_solver_options_layers.py (one for the global flag, one for the per-spoke {name}_solver_options_file); both pass.
• python -m pytest mpisppy/tests/test_solver_options_layers.py — 64/64 pass.
• ruff check clean on touched files.
• Manual: a Config with cfg.solver_options_file = \"\" no longer raises FileNotFoundError from shared_options.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.42%. Comparing base (265345b) to head (da61698).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #714      +/-   ##
==========================================
- Coverage   71.43%   71.42%   -0.02%     
==========================================
  Files         154      154              
  Lines       19463    19463              
==========================================
- Hits        13903    13901       -2     
- Misses       5560     5562       +2     

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