Fix empty set detection

A friendly error message here was a last minute addition,
but untested and as it turns out broken. Due to piping the loop executes
in a subshell and thus meta_prefix always retains its original value at
the time of check. Instead actually test for anything being written out.

yoink-or-update-emojipack.sh

@@ -56,7 +56,7 @@ cat "$cached_list" \
       fi
     done
 
-if [ "$meta_prefix" = "{" ] ; then
+if [ "$(wc -l "$META_ADD" | cut -d ' ' -f 1)" = "0" ] ; then
   echo "No emoji matched search criteria"
   rm "$META_ADD"
   exit 0