Zwave config NPE checks

Adds checks to avoid null pointer exception.
I thought about fixing it that way, but if you look at the call hierarchy you'll end up with another NPE in ZWaveConfiguration#doAction() (line 893), ZWaveConfiguration#doSet() (line 1007), and ZWaveConfiguration#getConfiguration() (line 168). So I thought it was easier to fix it here by returning an empty List. The call hierarchy for getAssociations() tests for null so returning null from that is an OK fix.