User mention autocompletion fails if avatar_urls array is not set

[glendaviesnz]

Describe the bug

Currently if a site is returning false for get_option( 'show_avatars' ) the @ mentions autocompletion fails due to a Cannot read property '24' of undefined error.

To reproduce
Steps to reproduce the behavior:

1. Set the show_avatars option to false for a given site
2. Open dev tools console
3. In the given site add a new post and try to mention a user by typing @joemaller
4. User autocomplete dropdown does not display and error can be seen in console

Expected behavior
If a site has show_avatars set to false then the user autocomplete dropdown should still display but without avatars

Screenshots

Desktop :

• OS: OSX
• Browser: Chrome
• Version 78.0.3904.70

Can be replicated on 6.8 and latest Gutenberg master.