Support files being ignored by babel configuration

[TomStrepsil]

Resolves #1469

Ensure that when babel.loadOptions returns null (which will be the case if a file is ignored by a babel configuration), this falls through to the guard to return the minimal options ({ plugins: [] }), thus preventing the error described in the issue.

For context, this was needed in a project that used a local babel preset, which had to be ignored to avoid a circular reference in babel. When using a typescript dangerfile, the shared babel config baulked, due to attempt to spread null.

This could be mitigated via DANGER_DISABLE_TRANSPILATION, but this then requires any local danger plugins etc to be in commonJs, which isn't ideal.

N.B. The babelify code was previously untested, and I have added bare-minimum test of the new functionality I have added, to simplify the review. However, I am happy to attempt to "scout rule" the remainder of the function in tests, if that will be welcome.

N.B. The mocked dangerfile (copied from prior-art in the same test file) had to be changed to a linted version - since despite tests passing locally otherwise, the appveyor build had a failure thus:

    - Expected  - 1
    + Received  + 2
    - import {a} from 'lodash'; a()
    + import { a } from 'lodash';
    + a();

p.s. running:

yarn build; node --inspect distribution/commands/danger-pr.js https://github.com/danger/danger-js/pull/1476

...as per the guide, has produced:

Processed 122 files (1.8s) (1 warning)

✔ No circular dependency found!

✨  Done in 5.72s.
Debugger listening on ws://127.0.0.1:9229/20b17684-51a1-4385-85ce-b6c9202e43ab
For help, see: https://nodejs.org/en/docs/inspector
Starting Danger PR on danger/danger-js#1476
You don't have a DANGER_GITHUB_API_TOKEN set up, this is optional, but TBH, you want to do this
Check out: http://danger.systems/js/guides/the_dangerfile.html#working-on-your-dangerfile
Jest tests passed :+1:

Unable to evaluate the Dangerfile
 Error: process.env.GITHUB_STEP_SUMMARY was not set, which is needed for setSummaryMarkdown
    at Object.setSummaryMarkdown (/danger-js/distribution/platforms/GitHub.js:133:23)
    at dangerfile.ts:89:31
    at step (dangerfile.ts:36:23)
    at Object.next (dangerfile.ts:17:53)
    at fulfilled (dangerfile.ts:8:58)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)


Danger: ⅹ Failing the build, there is 1 fail.
## Failures
Danger failed to run `dangerfile.ts`.
## Markdowns
## Error Error

process.env.GITHUB_STEP_SUMMARY was not set, which is needed for setSummaryMarkdown
Error: process.env.GITHUB_STEP_SUMMARY was not set, which is needed for setSummaryMarkdown
    at Object.setSummaryMarkdown (/git/danger-js/distribution/platforms/GitHub.js:133:23)
    at dangerfile.ts:89:31
    at step (dangerfile.ts:36:23)
    at Object.next (dangerfile.ts:17:53)
    at fulfilled (dangerfile.ts:8:58)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

### Dangerfile

--------------------------^

...which appears unrelated, and unclear how this can be resolved as a contributor?

If I run danger locally on this branch, no issues are present:

node distribution/commands/danger-local.js --base main

Danger: ✓ passed review, received no feedback.

[TomStrepsil]

N.B. These are needed to prevent an early-return within babel.

[TomStrepsil]

N.B. This was produced when running yarn lint - unclear why previous PRs did not produce the same? But without this, the CI fails.

[orta]

interesting, weird!

[orta]

Yeah, this makes sense to me 👍🏻

Thanks!