Explain when `schemas` and `database_schemas` would be empty within an `on-run-end` hook

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/reference/dbt-jinja-functions/on-run-end-context#schemas

### What part(s) of the page would you like to see updated?

Add some additional information explaining when the [`schemas`](https://docs.getdbt.com/reference/dbt-jinja-functions/on-run-end-context#schemas) or [`database_schemas`](https://docs.getdbt.com/reference/dbt-jinja-functions/on-run-end-context#database_schemas) would be empty within an `on-run-end` hook.

These are some _assumptions_ that would need to be fact-checked before adding them to the documentation:
- in the case of `dbt compile`, these context variables are empty because dbt hasn't built anything yet [[1](https://github.com/dbt-labs/dbt-core/issues/9301#issuecomment-1921581199)]
- if a `dbt run` (or `build`, `seed`, `snapshot`, etc.) doesn't succeed, then these context variables will be empty [[2](https://github.com/dbt-labs/dbt-core/issues/9301#issuecomment-2364325536)]

### Additional information

See https://github.com/dbt-labs/dbt-core/issues/9301 for background context.