Suppress "Windows Defender Antivirus did not find any threats..." notification on Windows 10

You can turn off Enhanced Notifications for Windows Defender:

1. Press Windows + i.
2. Click Update & Security.
3. From the Update & security sidebar, click Windows Defender.
4. Scroll to the Enhanced notifications section and select No.

If you do not see Enhanced notifications as an option (likely because you are running Windows 10 15042 or newer), include the following additional steps:

5. Click Open Windows Defender Security Center.
6. Click Virus & threat protection.
7. Scroll down and click Change notification settings.
8. Turn off Receive recent activity and scan results.

This will turn of successful updates and only notify you critical events.

This is essentially the same answer as the one by Ashton above, but Microsoft seems to have moved the option again:

1. In the "Windows Defender Security Center" (you can just Windows-search for it)
2. Enter the settings page (via the cog-symbol at the bottom left, for me)
3. Turn off the "Recent activity and scan results" checkbox