[TASK] Improve code coverage of Scheduler/FieldProvider

* Also fixed a small bug in Cli/Command.php

Releases: 6.1
Resolves: #44650
Change-Id: Ic9cc4ebb81a3926415223f6a7a8078198a39f1ab
Reviewed-on: https://review.typo3.org/17592
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring