Provide contextual fallback for dynamic blocks

[kraftbj]

When using blocks like Followers and Follow Me, previously, it reported that it would be empty in non-author contexts in the editor. In realty, this is only true if the blog-level profile is not enabled.

Proposed changes:

• Conditionally include the empty disclaimer to only when the site profile is disabled.

Other information:

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

Testing instructions:

• With AP set to only author profiles, attempt to add a Follow Me block set to Dynamic User. See the fallback text that mentions it'll be empty.
• With AP set to have the blog and author profiles, do the same as above. See the fallback text without the empty disclaimer.

[mattwiebe]

@kraftbj thanks so much for the PR! I updated it a bit to make proper use of i18n, because apparently adding a /* translators */ comment wasn't enough 🤦

[mattwiebe]

🎉