Avoid unnecessary string allocation when calculating length

[ghost]

Description

Hi, I am trying to optimize YARD a bit to speed up documentation runs on a large codebase.

The codebase is private, so I am using https://github.com/watir/watir/ as a smaller stand-in to report benchmarking numbers from https://github.com/SamSaffron/memory_profiler.

Test set-up

• Ruby 2.7.6 (installed via rvm on Linux)
• Watir version: 293bed2b57c
• MemoryProfiler version: 1.0.0
• YARD baseline: 0a55093
• Test command:
  • ruby-memory-profiler --no-color --retained-strings=500 --allocated-strings=500 --max=300 -o prof.log ./run-yard.rb stats
    • run-yard.rb is a smaller version of /bin/yard (MemoryProfiler was not able to run /bin/yard)
    • I am using the stats command to avoid memory exhaustion

Performance data

• I don't have concrete performance data because the stats command doesn't seem to trigger this code path
• On the private codebase, the loop body was executed ~375k times, so I think this optimization is worthwhile

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[lsegal]

Thank you for this PR!