Suppress "zsh: no matches found" error when $ZSH_CUSTOM has no files

The addition of `(.N)` enables the `NULL_GLOB` option which suppresses
the error output in question.

Reference: http://www.zsh.org/mla/users/2004/msg00621.html

oh-my-zsh.sh

@@ -51,7 +51,7 @@ for plugin ($plugins); do
 done
 
 # Load all of your custom configurations from custom/
-for config_file ($ZSH_CUSTOM/*.zsh) source $config_file
+for config_file ($ZSH_CUSTOM/*.zsh(.N)) source $config_file
 
 # Load the theme
 if [ "$ZSH_THEME" = "random" ]