vdl/java: fix generation of constants
vdl/java: fix generation of constants

Type zero numbers and fix zero-value case for constants:
type S struct {
    Err error
}
const SConst = S{}
The Err field should be initialized to its proper zero value.

Change-Id: I6eee176656a0f59f6d4347ea39d1ab1aeb2d0548

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23