portal: silently drop empty Matrix messages instead of erroring

[Deln0r]

Closes #186.

Problem

When a Matrix client sends a text/notice/emote with an empty body to a Discord-bridged room, the bridge relays it to Discord verbatim. Discord's API rejects the call with HTTP 400 / code 50006 ("Cannot send an empty message"), and the bridge surfaces an error notice back into the Matrix room. Ten empty messages therefore become twenty (ten originals plus ten error notices), which makes accidental empty sends a cheap room-spam vector.

Fix

Add an errEmptyMatrixMessage sentinel. On the MsgText/MsgEmote/MsgNotice branch of (*Portal).handleMatrixMessage, just after parseMatrixHTML resolves the final Discord content, check whether the content is whitespace-only AND there is no reply embed attached. If so, drop the message before making the API call:

if strings.TrimSpace(sendReq.Content) == "" && len(sendReq.Embeds) == 0 {
    go portal.sendMessageMetrics(evt, errEmptyMatrixMessage, "Ignoring")
    return
}

The metric call is still made with the Ignoring action so the checkpoint is recorded, but sendNotice is false for this error class, so no error message hits the Matrix room. Non-text content (images, files, audio, video) is unaffected: those paths attach a file separately and never reach this check.

Test plan

go build ./... and go vet ./... are clean (with libolm headers available; this matches what CI does).

No new test added because portal_test.go doesn't exist in this repo and a full end-to-end integration test would need a live Discord client. Happy to add a focused unit test for the empty-content branch if you want a regression guard.