nixos/home-assistant: allow null config value

While the documentation said to set this to null, in case an imperative
config was supposed to be used, this was not possible with the typing in
place.

nixos/modules/services/home-automation/home-assistant.nix

@@ -135,7 +135,7 @@ in {
     };
 
     config = mkOption {
-      type = types.submodule {
+      type = types.nullOr (types.submodule {
         freeformType = format.type;
         options = {
           # This is a partial selection of the most common options, so new users can quickly
@@ -244,7 +244,7 @@ in {
             };
           };
         };
-      };
+      });
       example = literalExpression ''
         {
           homeassistant = {