scrobbler: fix missing QNetworkAccessManager when session active.

Based on patch provided by tanjoodo, but simplified to create the
missing QNetworkAccessManager unilaterally.

This avoids a crash in the scrobbler when scrobbling is attempted with
an active session but no network manager (such as when the configured
internet connection is lost).

Differential Revision: https://phabricator.kde.org/D17140