fix(richtext-lexical): prevent invalid h0 heading nodes when all heading sizes are disabled (#16090)

## What

Fixes `HeadingFeature({ enabledHeadingSizes: [] })` creating invalid
`h0` heading nodes that corrupt stored data.

Closes #15899

## Why

When `enabledHeadingSizes` is an empty array, the heading markdown
transformer builds its regex from an empty alternation group:

```
pattern = "^()\\s"   // matches ANY leading whitespace
```

This regex matches any line starting with a space or tab. The empty
capture group produces `match[1] = ""`, so:

```js
tag = 'h' + match[1]?.length  // 'h' + 0 = 'h0'
```

An invalid `h0` heading node is created and persisted to the database.
If the HeadingFeature is later removed, these nodes cause a runtime
error (`parseEditorState: type "heading" not found`).

## Changes

Skip registering the heading markdown transformer when
`enabledHeadingSizes` is empty. No heading sizes enabled means no
heading markdown shortcuts should be active.

## Testing

- `npx tsc --noEmit` passes
- Lint passes
- With this fix, `HeadingFeature({ enabledHeadingSizes: [] })` registers
zero markdown transformers, so no heading conversion can occur