assert,util: fix TypeError on Maps with null keys

[bitpshr]

Fixes #64433.

assert.deepStrictEqual, assert.notDeepStrictEqual, and util.isDeepStrictEqual throw a TypeError on Maps with a null key in a specific shape: one map has a null key with an object value, the other has an object key with a deeply-equal value but no null key, and both maps are the same size.

It comes from mapObjectEquiv in lib/internal/util/comparisons.js, which only resolved primitive and null keys directly against the other map when array.length !== a.size. When the sizes match, the null key skipped that and reached the object comparator, which reads key.constructor with no null guard (and typeof null is 'object'). It only affects strict mode; loose already handled it and partial uses a different path.

The fix handles primitive and null keys unconditionally. I added a regression to test-assert-deep.js covering both the crashing case and the equal case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (abe2545) to head (484a3ff).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64441   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files         741      741           
  Lines      241376   241379    +3     
  Branches    45478    45486    +8     
=======================================
+ Hits       217843   217855   +12     
+ Misses      15087    15085    -2     
+ Partials     8446     8439    -7     

Files with missing lines	Coverage Δ
lib/internal/util/comparisons.js	99.71% <100.00%> (+<0.01%)	⬆️

... and 23 files with indirect coverage changes

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