Reactions: Don't display avatar when it's empty

[obenland]

Fixes #1101.

Proposed changes:

• Adds a fallback avatar URL to our default avatar, when there is none defined.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Create an actor with an empty avatar.
• Like a post on your test site.
• Make sure it's displayed with the default avatar.

After

[obenland]

@mattwiebe I didn't expect that running nom run build would result in changes for all blocks. Did I use the wrong command or is that just the way it works?

[mattwiebe]

@mattwiebe I didn't expect that running nom run build would result in changes for all blocks. Did I use the wrong command or is that just the way it works?

That's just the way it works, unfortunately.