Make uid null instead of undefined, when user is not logged in

src/database/session.ts

@@ -1,11 +1,10 @@
-
 export default class Session {
-   constructor(private _sessionid: string) { }
+   constructor(private _sessionid: string) {}
    get id() {
       return this._sessionid;
    }
    root: boolean = false;
-   uid: string = undefined;
+   uid: string = null;
 
-   subscriptions = new Map<string, (() => void)>();
+   subscriptions = new Map<string, () => void>();
 }