inputSpec should not be mandatory when inputSpecRootDirectory is set

[andreas-wirth]

Fix #16955 : inputSpec should not be mandatory if inputSpecRootDirectory is set
(@borsch @wing328 )

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[wing328]

@andreas-wirth thanks for the PR. can you please add a test?

[andreas-wirth]

@wing328 Added some unit tests, let me know if they fit your expectations

[MelleD]

Looks good so far :)

[andreas-wirth]

Any Feedback @wing328 ?

[wing328]

thanks for the PR, which has been merged