Ignore None consistently in URI template list expansions

[machines-6657]

Summary

Ignore None consistently when expanding list arguments. The 3.0.1 changelog
documents this behavior, but simple/reserved/fragment, query/continuation and
non-exploded semicolon expansions still include the literal text None.
For example, {?x} with x=["a", None, "b"] currently produces
?x=a,None,b; the corrected expansion is ?x=a,b.

The filters use is not None, preserving defined empty strings and numeric zero
and leaving the caller's list unchanged. Regression coverage includes all 16
operator/explode forms, empty/zero controls and mutation checks. Existing empty
collection and mapping behavior is outside this change.

Validation

• Against the unchanged base implementation, the added regression class has 22
  failing subtests; the mutation controls pass.
• Python 3.11, Windows: full suite, 61 tests and 102 subtests passed.
• Black, Flake8, mypy and git diff --check passed.
• The full upstream interpreter matrix and documentation build were not run.

AI assistance was used to investigate, implement and verify this change.