fix: InlineTextField renders empty for nullish values

[shiminshen]

Closes #1639

Description

InlineTextField was rendering the literal text "null" (or "undefined") in the contentEditable canvas whenever a text or textarea field with contentEditable: true received a nullish value. The cause is Node.replaceChildren(...nodes) coercing each non-Node argument via String(...) — String(null) === "null" and String(undefined) === "undefined" — so a defaultProp of subtitle: null, or a server response returning null for an unfilled optional field, persistently surfaced as null text in the editor until the user manually selected-all and deleted.

The richtext field type is unaffected because TipTap handles nullish content; this is specific to InlineTextField only.

Changes made

• Coerce value ?? "" before calling replaceChildren, in the same effect that syncs the contentEditable span's text.
• Widen the value prop type from string to string | null | undefined so the type matches what the text / textarea field transforms actually pass in (the data model allows nullish props).
• Add jest tests covering value={null}, value={undefined}, and value="Hello world".

How to test

Minimal repro from the issue:

const config = {
  components: {
    MyBlock: {
      fields: {
        subtitle: { type: 'textarea', contentEditable: true },
      },
      defaultProps: { subtitle: null },
      render: ({ subtitle }) => <p>{subtitle}</p>,
    },
  },
};

Insert MyBlock in the editor. Before: the canvas shows the text null where the subtitle would be. After: empty, like an empty string.

Or run yarn test --testPathPattern=InlineTextField from packages/core — 3 new tests cover the null / undefined / string cases.

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