Reset props when their attribute is removed or fails to parse (#230)

[razchiriac]

Fixes #230.

Problem

attributeChangedCallback ignored any change where the new value was empty or null
(guard (value || type === "method"), introduced in #146). Removing an attribute — or setting
one the transformer can't parse — left the React component rendering the stale previous prop
value, with no re-render at all.

Approach

Implements the direction @bmomberger-bitovi outlined in #230: lean on the Transform type's
ability (since @r2wc/core 1.3.1) for parse() to return undefined.

• Attribute removed (value === null) → the prop resets to undefined and the component
  re-renders ("just opting to use undefined if undefined").
• Empty-string values are now passed to the transformer instead of ignored (constructor and
  callback), so text="" yields "" rather than a stale value.
• The json transform catches parse errors and returns undefined, so an invalid value resets
  the prop instead of throwing/keeping the stale one.
• method keeps its existing always-parse behavior; transformers that never return undefined
  behave exactly as before for present, parseable values.
• attributeChangedCallback's signature now reflects the DOM (string | null).

Side effect worth calling out: assigning null/undefined to a stringify-backed property
previously updated internal state but never re-rendered (the removeAttribute it triggered was
swallowed by the same guard); it now re-renders with undefined.

Compatibility

Components whose attribute-driven props are typed as required will now receive undefined after
a removal — per the discussion in #230, those props should be typed optional. If you'd prefer to
ship this as a major, happy to retarget.

Tests

Five new tests in packages/core/src/core.test.tsx covering: removal resets the prop (+
re-render with { text: undefined }), empty value parses, invalid JSON resets instead of going
stale, a transformer returning undefined resets, and a set → update → remove → re-add
regression round-trip. Full nx run-many -t typecheck / eslint / test:ci / build green.