Avoids passing of useless parameter

[Hsilgos]

convert_from_jni functions use last parameter only for function overloading because
normally those functions are different by the return parameter. In all places where
function convert_from_jni is used the last parameter is nullptr which is C-casted to the desired type.
As godbolt demonstrates modern clang and gcc have to pass this parameter without any optimisation.
This can be improved if this parameter is empty object (i.e. an object of a class without any fields) and
it's passed by value. As godbolt shows in this case modern cland and gcc generate code as if there is no any parameter at all.