Properly initialize the CacheJail for sharing

[juliusknorr]

Replaces #16911

The set root path is expected to be an empty string instead of null when obtaining the jailed path in

server/lib/private/Files/Cache/Wrapper/CacheJail.php

Line 71 in 0487144

if ($this->getRoot() === '') {

This fixes two issues when sharing a complete group folder:

Search:

• Create a group folder and add some files to it
• Share the whole group folder with another user or guest account
• Search in the home folder for a file that exists in the groupfolder as user or guest
  💥 Filename not displayed properly and path is set wrong
  ✔️ File is displayed and linked properly

Activity:

• Create a group folder and add some files to it
• As admin share the whole group folder with a guest account
• As a guest user upload a file to the group folder
  💥 The activity app for the admin shows a broken entry
  ✔️ The activity app for the admin shows the correct file entry

[rullzer]

Master is no 18 development. If this needs to go into 17 please follow the normal backport procedures after merging.

[nickvergessen]

/backport to stable17

[nickvergessen]

/backport to stable16

[nickvergessen]

/backport to stable15

[nickvergessen]

Failures are unrelated

[backportbot-nextcloud]

backport to stable17 in #17060

[backportbot-nextcloud]

backport to stable16 in #17061

[backportbot-nextcloud]

backport to stable15 in #17062