'brackets': Allow unset ZSH_HIGHLIGHT_STYLES

Without this when bracket_color_size is 0,
    $(( (levelpos[$pos] - 1) % bracket_color_size + 1 ))
errors with division by zero.

highlighters/brackets/brackets-highlighter.zsh

@@ -76,7 +76,9 @@ _zsh_highlight_brackets_highlighter()
   # Now highlight all found brackets
   for pos in ${(k)levelpos}; do
     if (( $+matching[$pos] )); then
+      if (( bracket_color_size )); then
         style=bracket-level-$(( (levelpos[$pos] - 1) % bracket_color_size + 1 ))
+      fi
     else
       style=bracket-error
     fi