Preserve null values when converting nil containers to JSON

[x0Lazarus]

Summary

JSON conversion currently changes some nil slices and maps into empty arrays and objects. For example, objx.Map{"items": []interface{}(nil)} becomes {"items":[]} instead of {"items":null}. This loses the distinction between a null collection and an empty one; nil plain string-keyed maps and nil string slices already retain it.

Return nil from the existing cleanup helpers before allocating their output. Nil containers now remain null, while non-nil empty containers still become [] or {}. No new map types or recursive conversion paths are added.

The regression table covers nil, empty and populated containers directly and inside another map or slice. It checks JSON, MustJSON and the decoded Base64 output. All 54 cases pass; 15 fail before this change. The table cases run in parallel, with a per-iteration copy for older Go versions. The full suite and vet pass on Windows with Go 1.23.0 and 1.27.1. Five shuffled full-suite runs with the race detector also pass on Go 1.27.1.

Checklist

• Tests are passing: the go test -race ./... command used by task test passes locally with CGO enabled. Local validation covers Windows; the updated Ubuntu CI matrix is pending.
• Code style is correct: gofmt -d -s on the committed LF content of all Go files produces no changes, and go vet ./... passes.

[hanzei]

Thanks

[hanzei]

Can we t.Parralell() these tests?

[x0Lazarus]

Added t.Parallel() to the table cases and captured the loop variable for Go 1.20/1.21. The full suite and five shuffled race-detector runs pass on Windows.