fix: the substr function will return an empty string instead of false

[jvillafanez]

Description

With the php8 update, the substr function returns an empty string instead of false.
In this particular cases, the $mountPath check for false fails, causing the code to go through an unintended path. Information returned by the affected function (getUsersSharingFile) is wrong, and it can affect to other apps, such as the activity app (see linked ticket)

Related Issue

Related to owncloud/activity#1230

Motivation and Context

Wrong information is stored and shown to the user. This PR fixes it.

How Has This Been Tested?

• test environment:
• test case 1:
• test case 2:
• ...

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[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.