fix: correct README.md parsing without an existing Usage h2

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Transition mode clobbers existing README.md content after badges if there is no Usage h2 #2140
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

https://github.com/JoshuaKGoldberg/prune-github-notifications didn't have a ## Usage in its README.md, and instead of allowing all content until the next relevant h2 or end-of-file, the option was defaulting to undefined.

This reminds me that blockREADME hardcodes a ## Usage in there - that's not an assumption I want to keep. But removing that assumption can be a followup.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.81%. Comparing base (c0fa41b) to head (95df083).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2143   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files         129      129           
  Lines        5461     5475   +14     
  Branches      395      399    +4     
=======================================
+ Hits         5451     5465   +14     
  Misses         10       10           

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[github-actions]

🎉 This is included in version v2.26.7 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀