options: provide defaults for user and fullName

This fixes users not being present, even on the server. We have to
create a placeholder user, even if we're only going to ssh as root.

modules/options/homeVars.nix

@@ -11,7 +11,7 @@ in {
     fullName = mkOption {
       type = str;
       description = "your full name (used for git commits and user description)";
-      default = null;
+      default = "User";
     };
     email = mkOption {
       type = str;

modules/options/systemVars.nix

@@ -15,7 +15,7 @@ in {
     username = mkOption {
       type = str;
       description = "username for the home directory";
-      default = null;
+      default = "user";
     };
   };