Fix crash related to audioMuted and recentlyAudible properties in qml.

If the qml properties audioMuted or recentlyAudible are accessed / set
on a WebEngineView on application startup, the application will crash
due to accessing a null pointer (the web contents adapter is not
allocated yet).

Add guards against null pointer access, to prevent the crash.

Change-Id: I939be9afa093d1889024b2ff9308deb88b9316bf
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>