Default possibly unset variables to empty

tests/test-highlighting.zsh

@@ -49,7 +49,7 @@
 
 # Set up results_filter
 local results_filter
-if [[ $QUIET == y ]]; then
+if [[ ${QUIET-} == y ]]; then
   if type -w perl >/dev/null; then
     results_filter=${0:A:h}/tap-filter
   else

zsh-syntax-highlighting.zsh

@@ -188,7 +188,7 @@ _zsh_highlight_apply_zle_highlight() {
   integer first="$3" second="$4"
 
   # read the relevant entry from zle_highlight
-  local region="${zle_highlight[(r)${entry}:*]}"
+  local region="${zle_highlight[(r)${entry}:*]-}"
 
   if [[ -z "$region" ]]; then
     # entry not specified at all, use default value