Preserve explicitly empty URI authorities

[ac1982]

Fixes #78.

An explicitly empty authority is currently collapsed into an absent authority: urlparse('foo:///').unsplit() returns foo:/, and file:///tmp/a loses its // delimiter. Preserve the distinction between '' and None through URI/IRI construction, normalization, reassembly, and ParseResult conversions. from_parts() still treats an omitted host as an absent authority, while an explicitly empty host preserves //.

Add regression coverage for round trips, copies, normalization, string/byte conversions, and relative resolution, including controls for absent authorities. This addresses the empty-authority case independently of #155's authority-absent paths beginning with // and #156's dot-segment normalization.

Validation on Linux with Python 3.12:

• Initial regression run against the base: 18 failures, 4 passes.
• Final complete suite: 2,938 passed; 100% coverage, including 24 new cases.
• All pre-commit checks passed; HTML documentation build and all 57 doctests passed.
• Wheel and sdist builds passed.
• The project typing-coverage check passed (75% required). Standalone Pyright is not clean on the base: the same tool reports 56 errors / 6 warnings on the base and 55 errors / 6 warnings here. The removed error is the nullable byte-authority conversion fixed by this change; no new error messages were introduced.