Cannot unset share expiration date if admin has set a default-but-not-enforced expiration date

Steps to reproduce

1. Let the admin configure a "default expiration date for shares" under /settings/admin/sharing, e.g. to 7 days. DO NOT check the box to enforce the default expiration date.
2. Open the iOS Files app
3. Share a file and save the share
4. Click on ... > Details to edit the share
5. Click on "Set expiration date"
6. Click "clear" --> notice how the date is cleared
7. Save the share
8. Click ... > Details again to view the expiration date

Expected behaviour

The "Set expiration date" field should have no date set.

Actual behaviour

The "Set expiration date" field is set to the admin's default, e.g. 7 days in the future.

Screenshots

n/a

Logs

n/a

Reasoning or why should it be changed/implemented?

It's a bug. If the admin does not enforce it, the user should be able to set no expiration date. That's also how Android and web do it.

Environment data

iOS version: e.g. iPadOS 17.2

Nextcloud iOS app version: 4.9.6.1

Server operating system: Ubuntu

Web server: Apache

Database: MariaDB

PHP version: 8.2

Nextcloud version: 28.0.0

Start by reproducing the share-editing flow in the iOS Files app with a non-enforced server default expiration, then compare the behavior with Android and web. The fix is done when clearing the expiration remains unset after saving and reopening the share.