fetchers: Fix gist fetcher

[RytoEX]

Description

This PR contains two commits:

• fetchers: Update gist fetcher to check for usernames
• fetchers: Expand empty description check to include null

Motivation and Context

The first commit fixes checking gist URLs if they contain a username: https://gist.github.com/<username>/<gistId>. GitHub usernames can only contain alphanumeric characters and hyphens, and must be 1-39 characters.

The second commit fixes parsing gist JSON if the description is null instead of "". I don't know what causes this to happen, but I've been shown a gist that does this.

How Has This Been Tested?

Did some quick tests locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[prgmitchell]

LGTM