Fix acceptance test helper checking if expiry date available

[kulmann]

Description

This PR hardens the acceptance test helper which checks if an expiry date is disabled.

Fix was stolen from @pascalwengerter from #5939

How Has This Been Tested?

• acceptance test webUISharingPublicExpire/shareByPublicLinkExpiringLinks.feature:147 is not failing anymore

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[ownclouders]

Results for oC10SharingAccept https://drone.owncloud.com/owncloud/web/19936/14/1
The following scenarios passed on retry:

• webUISharingAcceptSharesToRoot/acceptShares.feature:108

[pascalwengerter]

Code LGTM, if CI is happy I'm too