Run 'dircolors' without '~/.dir_colors' if absent

Add an else condition to run $(dircolors) if the '~/.dir_colors' file
does not exist; this allows the use of default colors when the user has
not made any customizations.

alias.zsh

@@ -15,6 +15,8 @@ if (( $+commands[dircolors] )); then
   if zstyle -t ':omz:alias:ls' color; then
     if [[ -f "$HOME/.dir_colors" ]]; then
       eval $(dircolors "$HOME/.dir_colors")
+    else
+      eval $(dircolors)
     fi
     alias ls="$aliases[ls] --color=auto"
   else