fix: Ensure we don't get share by empty name

Signed-off-by: Carl Schwan <carl@carlschwan.eu>

Author: CarlSchwan

lib/private/Share20/Manager.php

@@ -1339,8 +1339,8 @@ public function getShareById($id, $recipient = null, bool $onlyValid = true): IS
 
 	#[Override]
 	public function getShareByToken(string $token): IShare {
-		// tokens cannot be valid local usernames
-		if ($this->userManager->userExists($token)) {
+		// tokens cannot be valid local usernames or empty
+		if ($token === '' || $this->userManager->userExists($token)) {
 			throw new ShareNotFound();
 		}
 		$share = null;