Expiration date can be set for last Manager of a Space

## Describe the bug
If you try to remove the last Manager from a space, ocis (correctly) rejects it with a "last manager can't be removed" error.

It is, however, possible to set an expiration date for the last Manager. The server will not respond with an error.

## Steps to reproduce
1. Send request to set an expiration date for the last Manager of a Space.

## Expected behavior
The request is rejected with an error stating that "no expiration can be set for last manager".

## Actual behavior
The server accepts the expiration date and sets it for the last manager.

## Setup
I used demo.owncloud.com to find this issue.

## Additional context
This issue was during the QA of https://github.com/owncloud/ios-app/pull/1435.