If null is permissible by your class, you can annotate the field or parameter with @Nullable. Guice recognizes any @Nullable annotation, like edu.umd.cs.findbugs.annotations.Nullable or javax.annotation.Nullable. —from the Guice site
To confirm chrylis's hunch above, Guice simply tests the name of the class.
// from core/src/com/google/inject/internal/Nullability.java
if ("Nullable".equals(type.getSimpleName())) {
return true;
}