CHOP-763: make sendReply() treat replyKey as optional.

It's not worth bothering to tweak reply LLSD or attempt to send it if the
incoming request has no replyKey, in effect not requesting a reply. This
supports LLEventAPI operations for which the caller might or might not care
about a reply, invoked using either send() (fire and forget) or request()
(send request, wait for response). This logic should be central, instead of
having to perform that test in every caller that cares.
The major alternative would have been to treat missing replyKey as an error
(whether LL_ERRS or exception). But since there's already a mechanism by which
an LLEventAPI operation method can stipulate its replyKey as required, at this
level we can let it be optional.