Set default for provider sync collection

lib/modules/rclone.nix

@@ -108,6 +108,7 @@ in {
         (name: provider: lib.mkOption {
           type = lib.types.attrsOf (mkSyncType provider);
           description = "The ${(provider config).displayName} synchronizations to set up.";
+          default = { };
         })
         syncProviders);
     };