test(core): cover nullsToUndefined and nullsToUndefinedShallow

[mariazuheros]

Adds unit tests for the null-to-undefined converters in packages/core, which were previously untested.

Covers both functions:

• nullsToUndefined: top level null and undefined, primitives left untouched, null values inside plain objects, recursion into nested objects, arrays left intact (their nulls survive since only plain objects are traversed), and the returned reference being the same mutated object.
• nullsToUndefinedShallow: top level null, top level replacement only, and no recursion into nested objects.

Tests follow the existing convert test style (vitest globals, colocated in tests).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}