fw-fanctrl: set default strategies (#426756)

nixos/modules/hardware/fw-fanctrl.nix

@@ -48,12 +48,11 @@ in
           };
 
           strategies = lib.mkOption {
-            default = null;
+            default = { };
             description = ''
               Additional strategies which can be used by fw-fanctrl
             '';
-            type = lib.types.nullOr (
-              lib.types.attrsOf (
+            type = lib.types.attrsOf (
               lib.types.submodule {
                 options = {
                   fanSpeedUpdateFrequency = lib.mkOption {
@@ -92,7 +91,6 @@ in
                   };
                 };
               }
-              )
             );
           };
         };