mobile/bind: handle null references from Java in Go

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.

Fixes golang/go#14228

Change-Id: I915d1f843c9db299d6910480f6d10dae0121a3b4
Reviewed-on: https://go-review.googlesource.com/19460
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/SeqTest.java

@@ -381,4 +381,12 @@ public class SeqTest extends AndroidTestCase {
     String got = n.getErr();
     assertEquals("want back the error message we set", want, got);
   }
+
+  public void testNullReferences() {
+    assertTrue(Testpkg.CallWithNull(null, new Testpkg.NullTest.Stub() {
+      public Testpkg.NullTest Null() {
+        return null;
+      }
+    }));
+  }
 }