ui: eslint: fix undefined check

'typeof' cannot return 'undefined' only the string '"undefined"', newer
eslint versions detect that as error

to fix it, directly check it for undefined instead of using typeof

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/manager6/storage/PBSEdit.js

@@ -365,7 +365,7 @@ Ext.define('PVE.panel.PBSEncryptionKeyTab', {
 			    } catch (e) {
 				return "Failed to parse key - " + e;
 			    }
-			    if (typeof key.data === undefined) {
+			    if (key.data === undefined) {
 				return "Does not seems like a valid Proxmox Backup key!";
 			    }
 			}