Fix Plink's handling of interactor_announce() blank lines.

I'd forgotten that the text-only branch of seat_antispoof_msg()
constructs a string from its input in the expectation that it's a
one-line message. So it was a mistake to put a \n at the start of the
string in interactor_announce() to get a blank line first.

Now interactor_announce() makes an extra call to seat_antispoof_msg to
show its blank line, and seat_antispoof_msg itself handles the
blank-line case specially.

utils/antispoof.c

@@ -12,7 +12,7 @@ void seat_antispoof_msg(InteractionReadySeat iseat, const char *msg)
          * unmodified as an unspoofable header.
          */
         put_datapl(sb, ptrlen_from_asciz(msg));
-    } else {
+    } else if (*msg) {
         /*
          * Otherwise, add enough padding around it that the server
          * wouldn't be able to mimic it within our line-length