fix: Remove allowEmptyValues since it leads to errors in the openAPI parser

spec/headers/hmac-cb-authentication.yaml

 in: header
 name: 'X-Callback-Authentication'
 required: true
-allowEmptyValue: false
 description: Der Message Authentication Code des übertragenen Callbacks
 schema:
   type: string

spec/headers/hmac-cb-timestamp.yaml

 in: header
 name: 'X-Callback-Timestamp'
 required: true
-allowEmptyValue: false
 description: Der Timestamp des Message Authentication Code.
 schema:
   type: integer