Make sure the relative path is always a string

[nickvergessen]

Not sure when this is triggered, but it happens:

server/lib/private/Files/Node/Folder.php

Lines 65 to 76 in 9d9f690

	public function getRelativePath($path) {
	if ($this->path === '' or $this->path === '/') {
	return $this->normalizePath($path);
	}
	if ($path === $this->path) {
	return '/';
	} else if (strpos($path, $this->path . '/') !== 0) {
	return null;
	} else {
	$path = substr($path, strlen($this->path));
	return $this->normalizePath($path);
	}

Exception

TypeError: Argument 3 passed to OC\Activity\Event::setObject() must be of the type string, null given, called in /var/www/html/apps/sharebymail/lib/ShareByMailProvider.php on line 323
  File "/var/www/html/lib/private/Activity/Event.php", line 377, in setObject
    public function setObject(string $objectType, int $objectId, string $objectName = ''): IEvent {
  File "sharebymail/lib/ShareByMailProvider.php", line 323, in publishActivity
    ->setObject('files', $fileId, $filePath);
  File "sharebymail/lib/ShareByMailProvider.php", line 257, in createShareActivity
    $userFolder->getRelativePath($share->getNode()->getPath())
  File "sharebymail/lib/ShareByMailProvider.php", line 757, in delete
    $this->createShareActivity($share, 'unshare');
  File "/var/www/html/lib/private/Share20/Manager.php", line 979, in deleteShare
    $provider->delete($share);
...
(13 additional frame(s) were not displayed)

Fix #14082

[nickvergessen]

/backport to stable15

[nickvergessen]

Sentry issue: NEXTCLOUD-NP

[rullzer]

makes sense.

[MorrisJobke]

Tested and still works 👍

[backportbot-nextcloud]

backport to stable15 in #14101