New-DbaAgentJobStep: Fix for using DatabaseUser

Type of Change

[x] Bug fix (non-breaking change, fixes #9378 )
[ ] New feature (non-breaking change, adds functionality, fixes # )
[ ] Breaking change (affects multiple commands or functionality, fixes # )
[ ] Ran manual Pester test and has passed (.\tests\manual.pester.ps1)
[ ] Adding code coverage to existing functionality
[ ] Pester test is included
[ ] If new file reference added for test, has is been added to github.com/dataplat/appveyor-lab ?
[ ] Unit test is included
[ ] Documentation
[ ] Build system

See issue for details.

I also changed the type of the parameter Database to string like we do it in all other commands. It was already used as string inside of the command.