Enhance/Refactor ServiceMethodInvoker.mapPrincipal()

[artem-v]

• Even if method is not secured - still apply principalMapper.map(context) (if mapper is present)

• Simplify

    return Mono.defer(() -> principalMapper.map(context))
        .switchIfEmpty(Mono.just(context.principal()));

to

return Mono.defer(() -> principalMapper.map(context))

Let client worry about what to return in the mapper. Don't make scalecube to "fallback" something if it was null.