Minor Bug Fix Suggestions

In trying to track down the cause of another issue, I noticed a few error messages in the console I think could be addressed in the next release.

1) Loading placeholder images from https://via.placeholder.com should use HTTPS so website using the plugin and have SSL can validate as secure.

2) The default avatar is missing from the URL called in the plugin. The image is not found at this URL: https://um.dev/wp-content/plugins/ultimate-member/assets/img/default_avatar.jpg