* feat(integrations): support optional integrations
By making integration optional, Astro can now ignore null or undefined Integrations instead of giving an internal error most devs can't read/won't understand.
This also enables optional integrations,
e.g.
```ts
integration: [
// Only run `compress` integration in production environments, etc...
import.meta.env.production ? compress() : null
]
```
* ci: add tests for optional integration
* docs: add changelog
