Fix false positives for valueless attribute selectors in selector-attribute-name-disallowed-list

[vkrol]

Clearly describe the bug

selector-attribute-name-disallowed-list doesn't handle valueless attribute selectors correctly.

Which rule, if any, is the bug related to?

selector-attribute-name-disallowed-list.

What code is needed to reproduce the bug?

[disabled] { }

What stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "selector-attribute-name-disallowed-list": ["disabled"]
  }
}

Which version of stylelint are you using?

13.8.0.

How are you running stylelint: CLI, PostCSS plugin, Node.js API?

CLI with stylelint "**/*.css" --config myconfig.json.

Does the bug relate to non-standard syntax (e.g. SCSS, Less etc.)?

No.

What did you expect to happen?

The warning was flagged.

What actually happened (e.g. what warnings or errors did you get)?

No warnings were flagged.

/cc @rletsin