Set default value if XDG variable not set

Co-authored-by: WATANABE Yuki <[email protected]>

share/initialization/default

@@ -10,7 +10,8 @@ cat <<__END__
 Welcome to yash!
 
 You are seeing this message because a user-specific initialization script is
-not found at $XDG_CONFIG_HOME/yash/rc or ~/.yashrc. The current session has been loaded with common
+not found at ${XDG_CONFIG_HOME:-$HOME/.config}/yash/rc or ~/.yashrc.
+The current session has been loaded with common
 settings that should be useful for many users. To suppress this message, copy
 a sample initialization script from ${yashdatadir}/initialization/sample
 to ~/.yashrc and make any customization in it if you want. For bare default