sd-varlink: skip IDL validation on synthetic empty terminator

sd-varlink: skip IDL validation on synthetic empty terminator

When a streaming method's callback returns 0 with no replies enqueued and
sd_varlink_set_sentinel() was passed NULL (POINTER_MAX), the framework
emits an empty parameters reply via sd_varlink_reply(v, NULL) as a clean
stream terminator. That call validated the empty parameters against the
method's output IDL — which always failed when the IDL declared any
mandatory output field, generating a spurious 'didn't pass validation'
warning even though the empty terminator is correct by construction.

Move the body of sd_varlink_reply() into a static varlink_reply_internal()
that takes a skip_validation flag. The public sd_varlink_reply() keeps
its existing behavior (validation enabled). Add a static
varlink_reply_terminator() helper that wraps
varlink_reply_internal(v, NULL, /* skip_validation= */ true), and route
varlink_dispatch_sentinel() through it for the synthetic empty terminator,
so both the synchronous dispatch path (varlink_dispatch_method()) and the
fiber path (varlink_fiber_entry()) skip validation in one place.

Without this, callers who want 'streaming method, possibly empty result'
semantics had to either fire a logically wrong 'NotFound'-style error
sentinel on empty (sysext.List, BootControl.ListBootEntries pattern) or
mark every output field NULLABLE just to placate the validator (the
io.systemd.Manager.EnqueueMarkedJobs approach), misrepresenting the
contract — exactly the tradeoff the FIXME in pcrlock.c flagged. With
this change, the empty terminator is invisible to the IDL validator and
the FIXME is no longer needed.

Co-developed-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>