set deactivated terminal handlers to empty functions instead of null

this allows to avoid console spamming with 'not a function' messages
after the connection to server was closed.

js/src/hterm.ts

@@ -75,9 +75,9 @@ export class Hterm {
     };
 
     deactivate(): void {
-        this.io.onVTKeystroke = null;
-        this.io.sendString = null
-        this.io.onTerminalResize = null;
+        this.io.onVTKeystroke    = function(){};
+        this.io.sendString       = function(){};
+        this.io.onTerminalResize = function(){};
         this.term.uninstallKeyboard();
     }