Replace unused dummy `SignalProducer` in unit tests with `SignalProducer.empty`.
Replace unused dummy `SignalProducer` in unit tests with `SignalProducer.empty`.

RyanTech/SwinjectMVVMExample,yoichitgy/SwinjectMVVMExample,yoichitgy/SwinjectMVVMExample,Swinject/SwinjectMVVMExample,RyanTech/SwinjectMVVMExample,Swinject/SwinjectMVVMExample,RyanTech/SwinjectMVVMExample