With the moveRange introduction, the MigrationSourceManager is calculating and initializing the max bound after its constructor is invoked.
It then can happen that getMigrationStatusReport gets called before the attribute is instantiated since the server status command is periodically invoked.

Objectives of the ticket:

• Change this line to _args.getMax().value_or(BSONObj())
• Put the following TODO on the line right before in order to not incur in the same problem when making min optional: // TODO SERVER-64926 do not assume min always present