Bug Fix: allow false in allowedClasses

[KevinSJ]

Summary

Prior to 2.4.0, allowedClasses can be false, this has been an undocumented feature.
Since 2.4.0, this behavior is broken. This PR reverted to the previous behavior where false is allowed in the allowedClasses attribute.

See #621 for more detail

What are the specific steps to test this change?

The following code is broken in the main branch of this base repo, but should work in the branch in this pr.

const sanitizeHtml = require('sanitize-html');

const ALLOWED_TAGS = [
    'li',
    'ul',
    'ol',
    'p',
    'h1',
    'h2',
    'h3',
    'h4',
    'h5',
    'h6',
    'img',
    'a',
    'div',
    'b',
    'table',
    'tbody',
    'tr',
    'th',
    'td'
];
const ALLOWED_SCHEMES = ['https', 'mailto'];

const sanitized = sanitizeHtml(`<ul class="should not sanitize"><li>Hello </li></ul>`, {
    allowedAttributes: { '*': ['*'] },
    allowedTags: ALLOWED_TAGS,
    allowedClasses: {
        ul: false
    },
    allowProtocolRelative: false,
    allowedSchemes: ALLOWED_SCHEMES
});
//expected <ul class="should not sanitize"><li>Hello </li></ul>
console.log(sanitized)

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

Other information:

[KevinSJ]

@boutell would you be able to merge this?