Prevent exception for a share file by inviting a new guest account

[JuliaKirschenheuter]

Before, steps to reproduce:

1. click Share on a file
2. enter an email address in the "Share with accounts and teams" fields
3. click "Invite guest", fill in a display name
4. click "Invite user and create share"
5. click "Save Share"
6. Then a "Failed to overwrite share target for notes" error message is thrown in Nextcloud log

This makes the customer uncertain although there is no real error.

After:

Do not throw exception if file was shared with a new guest account