Script no longer works if $XDG_CONFIG_HOME is unset

[arbyste]

I noticed that you recently added a section to make the script $XDG_CONFIG_HOME compliant, which is great, however because you use the set -u option at the top, it now causes an error to be thrown at line 35 if $XDG_CONFIG_HOME isn't defined, since you're trying to set an empty variable.

I could submit a pull request that removes the -u option from set, but I presume that you want to check for unset variables elsewhere in the script, so I'm not really sure what the best way to handle this is.