fix: handle empty null-prototyped objects

[alxndrsn]

Closes #5505

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: thrown error .expected with null prototype throws TypeError: value.toString is not a function #5505
• That issue was marked as status: accepting prs
• [i think so] Steps in CONTRIBUTING.md were taken

Overview

[linux-foundation-easycla]

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[JoshuaKGoldberg]

LGTM - a nice clean fix with a clean unit test. Thanks!

Will leave open a bit in case another maintainer wants to take look too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (f4e7e54) to head (672e8e6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5506   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files          57       57           
  Lines        4384     4385    +1     
  Branches      846      847    +1     
=======================================
+ Hits         4107     4108    +1     
  Misses        277      277           

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