fix: update json export to properly parse tagName elements

[madalynrose]

Details

Our Assessment JSON export mechanism surfaces the rule id and description for each requirement. The description originates as JSX and then gets converted into a string. This works fine for the majority of our requirements, where the JSX props contain children that mostly include innerText. This is not the case for several requirements that include Markup.Tag tags.

For example, the Semantics Assessment's "Header Attribute" Requirement includes two Markup.Tag tags for td and th that do not have children.

const headersAttributeDescription: JSX.Element = (
    <span>
        The <Markup.CodeTerm>headers</Markup.CodeTerm> attribute of a <Markup.Tag tagName="td" />{' '}
        element must reference the correct <Markup.Tag tagName="th" /> element(s).
    </span>
);

When our JSON exporter encounters tags without children, it appends "undefined" to the string:

{
   "requirementKey": "headersAttribute",
   "requirementDescription": "The headers attribute of a undefined element must reference the correct undefined element(s)"
 } 

This fix adapts our description builder to look for the tagName property if children does not exist.

{
   "requirementKey": "headersAttribute",
   "requirementDescription": "The headers attribute of a td element must reference the correct th element(s)"
}

Motivation

Addresses issue #6122

Pull request checklist

• Addresses an existing issue: english a/an in error text #6122
• Ran yarn null:autoadd
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS

[katydecorah]

Nice fix, @madalynrose!