Fix str() scrubbing literal "None" from names when empty_attribute_default is None

[derek73]

Summary

With empty_attribute_default = None, __str__ formatted the template with raw attribute values and then deleted every occurrence of "None" from the result — a scrub (dating to PR #45, 2016) that couldn't distinguish the interpolated None from real name text containing the same substring:

c = Constants()
c.empty_attribute_default = None
str(HumanName("Nonez Smith", constants=c))   # 'z Smith'

The fix substitutes '' for falsy attributes before str.format() runs and deletes the scrub outright, mirroring how initials() was fixed for the same bug family in 1.3.0. This also removes the latent corruption for any custom empty_attribute_default string (e.g. "N/A") appearing in real name text.

Closes #254

Testing

• Two regression tests in tests/test_output_format.py: a name containing "None" as a substring and a name piece that is exactly None (both watched failing with 'z Smith' before the fix)
• Full suite: 1472 passed, 22 xfailed; mypy and ruff clean
• Release log entry added under 1.3.1 - Unreleased

🤖 Generated with Claude Code

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.69%. Comparing base (770ed0a) to head (c8b245a).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #277   +/-   ##
=======================================
  Coverage   97.69%   97.69%           
=======================================
  Files          13       13           
  Lines         912      912           
=======================================
  Hits          891      891           
  Misses         21       21           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.