ADM with no programme metadata makes Scene go blank

[firthm01]

When importing ADM with no high-level metadata, the behaviour should be to stick to the initial single default programme and remain in auto mode, so everything is added to a default programme.

However, what actually happens is that the Scene goes blank. This is because the scenestore generated by the PendingStore class is empty and when we set this to be active, it overwrites the store with a blank store that has no programmes. In turn, this setting of the store also disables auto mode.

PendingStore should only set the store if it has something to set.

[firthm01]

Can be tested with the EBU test file channels_mono_adm.wav