eth/filters: remove use of event.TypeMux for pending logs

[fjl]

This fixes the following staticcheck warnings:

eth/filters/api.go:56:13: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
	mux       *event.TypeMux
	           ^
eth/filters/filter.go:35:14: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
	EventMux() *event.TypeMux
	            ^
eth/filters/filter_system.go:92:13: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
	mux       *event.TypeMux
	           ^
internal/ethapi/backend.go:46:14: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
	EventMux() *event.TypeMux
	            ^
accounts/abi/bind/backends/simulated.go:79:42: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
		events:     filters.NewEventSystem(new(event.TypeMux), &filterBackend{database, blockchain}, false),
		                                       ^
accounts/abi/bind/backends/simulated.go:467:38: SA1019: event.TypeMux is deprecated: use Feed  (staticcheck)
func (fb *filterBackend) EventMux() *event.TypeMux { panic("not supported") }
                                     ^

[karalabe]

This looks really weird. Why do we have this function?

[fjl]

We do this because returning a nil subscription is not allowed. nullSubscription returns a subscription that never sends anything.

[fjl]

@karalabe PTAL

[karalabe]

LGTM