core: fix NMManager in private-bus-only case
NMManager was failing to initialize if there was only a private bus, despite the fact that this is exactly the use case that the private bus was added for. The only other potentially-failing code in nm_manager_new() was adding prop_filter to the D-Bus connection, but this can't really fail, so just assert that it doesn't. And now, nm_manager_new() always succeeds, so update the caller for that.
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