Hide the emulated cursor when target is null

Makes it easier to understand what happens when a real element isn't
passed as a target to updateVisibility(). Also makes the code more
robust to future changes.

Co-authored-by: Alex Tanskanen <aleta@cendio.se>
Co-authored-by: Niko Lehto <nikle@cendio.se>

kasmweb/core/util/cursor.js

@@ -209,6 +209,9 @@ export default class Cursor {
     // (i.e. are we over the target, or a child of the target without a
     // different cursor set)
     _shouldShowCursor(target) {
+        if (!target) {
+            return false;
+        }
         // Easy case
         if (target === this._target) {
             return true;