src: do not pass nullptr to std::string ctor

[ckerr]

Fixes this error that I hit when working on a Chromium bump today in Electron:

2025-03-07T01:05:01.8637705Z ../../third_party/electron_node/src/debug_utils.cc(257,12): error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
2025-03-07T01:05:01.8638267Z 257 | return nullptr;
2025-03-07T01:05:01.8638481Z | ^~~~~~~
2025-03-07T01:05:01.8638700Z 1 error generated.

This code has been around for awhile in Node.js, so the new build failure is probably an Electron issue, e.g. CI config.

Even so, upstreaming a fix feels worthwhile since building a std::string from nullptris undefined behavior in C++20 and is prohibited as of C++23.

This PR just replaces the return nullptr; with a return {}; in a function that returns a std::string.

CC @codebytere

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.21%. Comparing base (117c1ca) to head (c8a3a3d).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #57354   +/-   ##
=======================================
  Coverage   90.21%   90.21%           
=======================================
  Files         630      630           
  Lines      185304   185304           
  Branches    36261    36266    +5     
=======================================
+ Hits       167178   167179    +1     
- Misses      11082    11084    +2     
+ Partials     7044     7041    -3     

Files with missing lines	Coverage Δ
src/debug_utils.cc	60.90% <ø> (ø)

... and 23 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/65628/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/65631/

[nodejs-github-bot]

Landed in 9a9a45a