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

@@ -1396,10 +1396,11 @@ public function getSharesByPath(\OCP\Files\Node $path, $page = 0, $perPage = 50)
 	 * @throws ShareNotFound
 	 */
 	public function getShareByToken($token) {
-		// tokens cannot be valid local user names
-		if ($this->userManager->userExists($token)) {
+		// tokens cannot be valid local usernames or empty
+		if ($token === '' || $this->userManager->userExists($token)) {
 			throw new ShareNotFound();
 		}
+
 		$share = null;
 		try {
 			if ($this->config->getAppValue('core', 'shareapi_allow_links', 'yes') === 'yes') {