QtNetwork (examples) - update broadcastreceiver too

... as soon as we updated the 'sender' example. Changes are purely cosmetic
though - 'explicit', nullptrs and local variables instead of redundant
data-members (QPushButton).

Task-number: QTBUG-60628
Change-Id: I572219da9d2a0ced07d94efb6f52f00b5a9c546d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>