Return nil for empty parse_max input

[OskarEichler]

Summary

parse_max('') calls length on nil because an empty string has no first line. Return nil before attempting prefix parsing.

Reproduction

Run after require 'fugit':

p Fugit.parse_max('') # now nil; previously NoMethodError
p Fugit.parse_max('', cronish: true) # nil

Verification

• rbenv Ruby 4.0.6, TZ=UTC. Master baseline and this individual patch: 1,524 existing tests, 3,379 assertions, zero failures, one existing pending test.
• 5 focused checks for this change, run outside the repository. The combined release-based branch passes 1,517 existing RSpec examples, zero failures, one pending, plus 998 focused checks.
• Also checked the combined master changes with unmodified registry EtOrbi 1.4.1/Raabro 1.5.0: 1,524 tests/3,379 assertions, zero failures, one pending, and 997 focused checks. The extra blank-line check requires the independently fixed Raabro empty-input behavior; it is not claimed to pass on unmodified Raabro.
• Individual/release checks used the same versions of EtOrbi/Raabro with their separately proposed fixes. Syntax and whitespace checks pass. Combined source has the same 18 pre-existing Lint warnings, with no new warnings. Release gem packaging checked.

Compatibility and limitations

Empty input now returns nil consistently with unsuccessful parsing. No valid-input behavior or signature changes.

No dependency/version upgrades, test additions or test modifications are included. The task's no-new-tests constraint was followed; reproduction/check scripts stayed outside the repository. Other Ruby versions and platforms await upstream CI. No production execution was performed. Prepared with Codex assistance.