Made the initiator.email optional and added a new validation.
Added a new regex pattern to string fields to prevent them from consisting only of whitespace.
Security-concept-update-needed: false
JIRA Work Item: [STACKITRMA-677](https://jira.schwarz/browse/STACKITRMA-677)
The subject claim may contain underscore characters which are not allowed by the schema yet. Therefore, an adjustment of the regex in the schema has been made to allow it.
Security-concept-update-needed: false.
JIRA Work Item: STACKITALO-437
