Quote $XDG_DATA_HOME when setting error file

I'm not sure if this is actually a bug in fish, but (test -d) with no argument returns true, and nonexistent environment variables return an empty string, so (test -d $XDG_DATA_HOME) returns true when $XDG_DATA_HOME isn't set.

Prior to making this change, I would get an error from dirname when opening a shell and an error from fish when cd'ing.

bin/autojump.fish

@@ -13,7 +13,7 @@ complete -x -c j -a '(autojump --complete (commandline -t))'
 # set error file location
 if test (uname) = "Darwin"
     set -x AUTOJUMP_ERROR_PATH ~/Library/autojump/errors.log
-else if test -d $XDG_DATA_HOME
+else if test -d "$XDG_DATA_HOME"
     set -x AUTOJUMP_ERROR_PATH $XDG_DATA_HOME/autojump/errors.log
 else
     set -x AUTOJUMP_ERROR_PATH ~/.local/share/autojump/errors.log