fix edge case bug in parsers where avatarUrl is null

[CollinBeczak]

Resolves: maproulette/maproulette3#2205

Solution: Make users.avatar_url an option[string] instead of a set string, and then give it an empty string fallback if the column has no value.
I tested this by removing the image of my user and re-rendering the comments:

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[ljdelight]

Updating a parser works, but there are more in the code that hits the avatar_url. Why not create a migration to have the db require not-null avatar URL and set existing nulls to the current default (/assets/images/user_no_image.png)? That was the preferred in the bug writeup maproulette/maproulette3#2205

[CollinBeczak]

I can do that, all of the parsers already have avatar_url as an option, so i just converted the remaining ones that i could find to options as well. I'll hold off on merging this one, and look into adding a migration.