Improve TemplateHaskell code to support empty objects

Currently, an empty objects will generate warnings as the arguments of
various functions are unused. By adding conditional code for this, we
can support generation of empty objects, e.g. like needed in Rpc code.

Additionally, the patch also converts RpcCallVersion to THH, now that
it can build it. We change the serialisation for this (from JSNull to
JSObject []), but this shouldn't matter as this is not used in
production.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>