fix: set Root start/end to null when fragment contains only whitespace

[ryanatkn]

This fixes a small parser bug where the AST root start/end values are invalid (start > end) for components with script and style tags but no markup.

before:

	"start": 54,
	"end": 53,

after:

	"start": null,
	"end": null,

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: a81eabc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17125

[ryanatkn]

alternatively, less explicit/clear and more defensive:

if (this.root.fragment.nodes.length) {
	// ...

	if (start <= end) {
		this.root.start = start;
		this.root.end = end;
	} else {
		// @ts-ignore
		this.root.start = this.root.end = null;
	}
} else {
	// @ts-ignore
	this.root.start = this.root.end = null;
}

[ryanatkn]

before the fix:

	"js": [],
	"start": 54,
	"end": 53,
	"type": "Root",