Add empty immutable graph factories

[JakeWharton]

I'm having to do things like

ImmutableGraph.copyOf(GraphBuilder.directed().build<String>())

in order to practice the null-object pattern rather than use null for default values of graph properties.

It would be nice to have empty() or of() factories for an immutable version of a graph/value graph/network for this reason.

[netdpb]

I think the intended pattern is the following, which is still not as simple as it could be for empty graphs, but it's simpler than what you have.

GraphBuilder.directed().<Foo>immutable().build() // Java
GraphBuilder.directed().immutable<Foo>().build() // Kotlin

[JakeWharton]

Ah, I don't have an immutable(). I think I need a version bump! Thanks.