Thank you for posting your solution so that others experiencing the same thing can easily reference this!
Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept "the answer. Accepted answers show up at the top, resulting in improved discoverability for others.
Issue: We are working on migrating from an Azure MariaDB to SQL Server on an Azure VM. The process was working until we upgraded MariaDB from 10.6.x to 10.11.x. Now we get an error stating, "Error occurred while collecting data". Additional error in output pane states, "Object cannot be cast from DBNull to other types.
Solution: SQL Server Migration Assistant (SSMA) is not certified for MariaDB as a source database for the migration tool. This could be the root cause of the error you’re experiencing. Downgrading your MariaDB installation to 10.6.x, which was working previously, could indeed resolve the issue.