Preserve null entry order in repeated nodes

[fhgffy]

Building { item: ["first", null, "last"] } currently emits the empty node before first, changing the array's order. Repeated processing instructions have the same problem.

Append null entries to the repeated-node buffer in place. The existing oneListGroup representation is preserved.

Validation: five new regressions fail on the base revision; all seven focused cases and the complete suite pass (143 specs, 0 failures, 1 existing pending). Both bundles build, ESM/CommonJS runtime checks pass, and the new spec passes ESLint.

Direct source lint has the same four existing errors as the base. The current lint script references an absent benchmark directory, and test-types references an absent spec/typings/typings-test.ts.