webadmin,userportal: Fix AsyncQuery related NPE

AsyncQuery no-argument constructor is used throughout both
UiCommon and GWT UI code.

This constructor leaves 'asyncCallback' reference as null,
among other fields. It's therefore assumed that this reference
will be set after AsyncQuery constructor.

However, in some places, this is not the case. For example,
AboutPopupPresenterWidget invokes public GetConfigurationValue
query without setting 'asyncCallback' reference, and Frontend
query success handler fails on NPE with 'callback.getDel()'
returning null.

This patch modifies AsyncQuery no-argument constructor to fix
the above mentioned issue, using null object singleton that
represents an empty (no-op) query callback.

Change-Id: I57a3794c70b6c48e58285cf6366bcbfd4e84665d
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>