Apply convert to defaults and preserve explicit null writes

[DmitrySharabin]

Summary

Two related defaults bugs in Prop.js:

• A resolved default skipped spec.convert, so { default: 5, convert: v => v * 2 } read as 5 instead of 10. get() now applies convert after parse, matching what a user write goes through.
• Explicit el.v = null was overridden by the default. get() no longer treats null the same as undefined. To keep removeAttribute resetting to default, set() collapses null-from-attribute to undefined.

Unlocks the two pinned tests in the Defaults group (Default passes through convert, null is preserved on a prop with a default); suite goes 93 → 95 passing.

Test plan

• npm test — Defaults group fully green
• Confirm reflection.js removeAttribute collapses a reflected prop to its default still passes

[DmitrySharabin]

Note: this PR was auto-merged by GitHub as a side effect of a restack error — the head branch was force-pushed such that it became an ancestor of the base, which GitHub interprets as "already merged." The original commit 522905e was preserved by cherry-picking it onto rollback-signals as commit a599e8f, so the change is correctly landed there.