Describe the change
The default value for the mime_db file must be None in order for it to use the default database file in the /usr tree. Defaulting to an empty string "" causes Magic to throw an exception due to a missing file at that empty path.

This likely only affects users that need to generate the config file via yaml formatters that are unable to convert None to null.

I did not create an issue. Let me know if you'd like one created and linked to this PR.

Describe testing procedures
Start the backend container with the tasting.mime_db setting deleted from the backend config file. The container will crash. Pull the fixed image and start a new container, still with the deleted setting. The container should not crash, and should start up normally.

Sample output
N/A

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings