fix(replaceOrAppend): treat only null/undefined as a missing new item

[mahirhir]

Summary

replaceOrAppend uses a truthiness check to decide whether a new item was passed:

if (!array && !newItem) return []
if (!newItem) return [...array]

Because !newItem is true for any falsy value, a falsy newItem such as 0, '', false, or NaN is treated as "no item" and silently dropped instead of being replaced or appended:

replaceOrAppend([1, 2, 3], 0, n => n === 2)      // got [1, 2, 3], expected [1, 0, 3]
replaceOrAppend([1, 2, 3], 0, n => n > 100)      // got [1, 2, 3], expected [1, 2, 3, 0]
replaceOrAppend([true], false, x => x === true)  // got [true],    expected [false]

The sibling replace already handles this correctly because it checks newItem === undefined, and toggle checks item === undefined, so both treat falsy values as valid items. replaceOrAppend is the only one of the three that uses a truthiness check.

The fix narrows the guard to newItem == null. This still covers null and undefined (the existing "missing item" tests pass null, so that behavior is preserved) while no longer swallowing falsy values. == null is the same null/undefined idiom already used elsewhere in the codebase, for example in select, clamp, and concat.

Related issue, if any:

None.

For any code change,

• Related documentation has been updated, if needed
• Related tests have been added or updated, if needed
• Related benchmarks have been added or updated, if needed
• Release notes in next-minor.md or next-major.md have been added, if needed

Does this PR introduce a breaking change?

No