Make empty-duration handling explicit for parser compatibility

[OskarEichler]

Summary

Make the existing empty-duration-as-zero behavior explicit in Duration.parse, including whitespace-only input. This preserves the contract used by Rufus without depending on a parser treating a failed match as successful.

This is a compatibility follow-up to floraison/raabro#13. With released Raabro 1.5.0, Fugit already returns a zero duration for empty input. With the proposed Raabro correction, that accidental acceptance disappears and Rufus's existing parse_duration example fails. Fugit should express its own accepted empty-duration policy.

Reproduction

require 'fugit'
Fugit::Duration.parse('').to_sec # intended existing result: 0

Using the corrected Raabro branch without this change returns nil instead. Rufus::Scheduler.parse_duration('') then raises, contradicting the existing Rufus expectation of 0.0.

Verification

• rbenv Ruby 4.0.6, TZ=UTC. Existing Fugit master suite: 1,524 tests, 3,379 assertions, zero failures, one pending.
• Sixteen external checks cover empty/space/newline inputs, plain/ISO/stricter options and preservation of original input/options. Invalid nonempty input still returns nil.
• Combined release-based Fugit changes: 1,517 existing examples, zero failures, one pending, plus 998 focused checks with the restored empty-duration contract.
• Rufus's existing parser suite: 37 examples, zero failures, including its empty-string expectation. The full 328-example Rufus integration suite is still running; no full-suite pass is claimed here.
• Packaging, syntax and whitespace checked. Combined source retains the same 18 pre-existing Lint warnings; no new warnings.

Breaking changes and limitations

No intended breaking change: this preserves released behavior across the parser correction. Empty durations remain zero; invalid nonempty durations remain invalid. No dependency/version upgrades or test additions/modifications are included. Reproduction scripts stayed outside repositories under the task's no-new-tests policy. Other Ruby/platform combinations await upstream CI. No production execution. Prepared with Codex assistance.