Make Subject required by default

[oruebel]

nwb-inspector has upgraded the check-subject-exists to a warning/error to ensure compliance with DANDI. We should:

• Update the tests to generate a Subject for all NWB-files that are being checked by the inspector and remove the --ignore=check-subject-exists from the workflows that was added in Ignore subject exists check in validation #289
• Update NWBFile to require subject metadata as input by default with the option to set it to null to explicitly state that a subject should not be created. This is to encourage creation of a valid Subject while still permitting to not create a Subject if the subject is unknown.