fix bug when using where on an empty collection
fix bug when using where on an empty collection

ReactiveX/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,ReactiveX/RxCpp,Reactive-Extensions/RxCpp,alexeymarkov/RxCpp,alexeymarkov/RxCpp