The Seq Java class has a special case for null references. Expand
the special case to Go so that null references from Java are properly
translated to nil.
Fixesgolang/go#14228
Change-Id: I915d1f843c9db299d6910480f6d10dae0121a3b4
Reviewed-on: https://go-review.googlesource.com/19460
Reviewed-by: David Crawshaw <crawshaw@golang.org>