Only render GFM when the element exists

What does this MR do and why?

renderGFM can get called with an undefined element. To fix the resulting errors, we return early when the element is not defined.

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Work Items

1. Open your browser console
2. Go to the page of an issue
3. Add a task without a description
4. See the error

Issues

1. Open your browser console
2. Create a new issue with an empty description
3. See the error in the console

MR acceptance checklist

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