fix: explicitly check for null props in serializeSignals

[lukasbachlechner]

Changes

Following up on @ph1p's changes, we discovered our pages were suddenly failing. Example error message:

TypeError [Error]: Cannot convert undefined or null to object
      at Function.keys (<anonymous>)
      at serializeSignals (file:///Users/lukasbachlechner/development/astro/packages/astro/test/fixtures/preact-component/dist/renderers.mjs:49:44)
      at Object.renderToStaticMarkup (file:///Users/lukasbachlechner/development/astro/packages/astro/test/fixtures/preact-component/dist/renderers.mjs:146:3)
      at renderFrameworkComponent (file:///Users/lukasbachlechner/development/astro/packages/astro/test/fixtures/preact-component/dist/chunks/astro/server_B-ybt-25.mjs:1263:53)
      at async renderComponent (file:///Users/lukasbachlechner/development/astro/packages/astro/test/fixtures/preact-component/dist/chunks/astro/server_B-ybt-25.mjs:1441:10)
      at async renderChild (file:///Users/lukasbachlechner/development/astro/packages/astro/test/fixtures/preact-component/dist/chunks/astro/server_B-ybt-25.mjs:904:13) {
    id: 'src/pages/signals.astro'
  }

Turns out that we need to explicitly check if props[key] !== null because typeof null returns "object", so Object.keys(props[key]) threw this error. Added this line to the isPropObject and also added a test case for it.

Testing

Test added in astro/test/preact-component.test.js.

Docs

No user-facing changes, so none needed.

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[matthewp]

Wonderful, thank you!

[matthewp]

There's a problem with our smoke tests, unrelated to your changes. Working on fixing that.