fix(taskAttachment): prevent NPE for deletion of attachments with empty or nonexisting taskId

[PHWaechtler]

related to #3545

• adds checks for empty or nonExistent taskIds when deleting a taskAttachment

Behaviour is now:
Deleting a task attachment providing an empty taskID:

• it will try to find the attachment via attachmentId. If no attachment found, exception is returned stating no attachment exists
• if an attachment with the given attachmentId exists, it will delete this attachment.
• no attempt is made to update the nonexisting task, avoiding the NPE

Deleting a task attachment with an ID of a task that has already been completed:

• if the attachment still exists, the attachment will be deleted
• no attempt is made to update the nonexisting task, avoiding the NPE

Note:
Due to this existing implementation, the NullValueException thrown in the Delete command will always be returned as an InvalidRequestException as the API response. I am assuming this is expected behaviour, but lmk if that should be adjusted in any way.

[PHWaechtler]

Lmk if we prefer not to introduce this dependency for some reason, can also just do the StringUtils.isNotBlank check manually.

[PHWaechtler]

Context:
I changed the name for this test case because to me it looks like the exception thrown here is thrown because the attachment was never created, not because the taskId is null (also made the exception assertion more specific for this reason)

[PHWaechtler]

Suggested follow up:
Based on my understanding of the issue, I think it would also make sense to look into prohibiting the creation of attachments with empty taskId and null (or empty) processInstanceId. At the moment, this is still allowed because the creation only checks for the taskId not being null. I can open a follow up ticket if you agree that this makes sense