dataengines/mpris2: Fix empty player condition
m_proxies does not always contain all players, and a player is added to m_proxies only when it has a valid proxyPid. But the container is ensured to be added to one of m_playing, m_paused and m_stopped.
So we need to ensure m_playing, m_paused and m_stopped are all empty
before emitting playerListEmptied()
.
BUG: 447169 FIXED-IN: 5.24