Ensure empty extended config throws a diagnostic

[marcins]

↪️ Pull Request

Small fix for a bug where an empty extended config file would result in an error message that didn't point to the root cause, this was because the error was an AssertionError and the error handling tried to process it as a Diagnostic.

💻 Examples

Before:

❯ parcel build
TypeError: Cannot read properties of undefined (reading 'message')
    at new ThrowableDiagnostic (/Users/mszczepanski/Work/atlassian/data-playground/node_modules/@parcel/diagnostic/lib/diagnostic.js:124:26)
[...]

After:

❯ parcel build
@parcel/core: packages/parcel-config-broken/index.json can't be empty

🚨 Test instructions

Extend an "empty" config file (i.e. index.json contains {})

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs