test: add util.inspect test for null maxStringLength
Add test case to cover currently-uncovered code.
Refs: https://coverage.nodejs.org/coverage-39a7f7663e8f70fc/lib/internal/util/inspect.js.html#L333
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines