Update stylelint rule no-empty-source to be consistent with Drupal's CSS standards

The only code that fails this test is Stark's layout.css, which is completely empty. I wasn't sure what we wanted to do about that, so I left it alone. This tiny patch has nothing except for changing the no-empty-source rule to true.

This patch is dependent on #2865971-41: Use stylelint as opposed to csslint in core.

The problem with changing the same file in different patches is that there needs to be a reroll each other one that gets committed.

Checking patch core/.stylelintrc.json...
error: while searching for:
    "function-name-case": null,
    "function-whitespace-after": null,
    "length-zero-no-unit": null,
    "no-empty-source": null,
    "no-unknown-animations": true,
    "number-leading-zero": null,
    "number-no-trailing-zeros": null,

error: patch failed: core/.stylelintrc.json:19
error: core/.stylelintrc.json: patch does not apply

You know I'm always happy to provide a re-roll.

fixed the patch for 8.4.x

Thanks @Jo Fitzgerald, @harsha012, @BrightBold, and @alexpott

I'm RTBC #8. I tested the linter with and without the CSS changes and reviewed the changes.

I'm a bit unsure why we have an empty stylesheet and half tempted to recommend removing it but I'll let a committer help decide it's fate.

fixed as per the #14 comment

fixed as per #16

@harsha012 just need a new line and should be made against 8.6.x, it's not a bug so we don't need to backport it. And the line could probably end with "purposes."

Add new line.
Change "purpose" to "purposes".

Thanks!