Pass empty params as null (#115)

packages/api-provider/src/coder/json/encodeObject.ts

@@ -10,6 +10,8 @@ export default function encodeObject (self: RpcCoderState, method: string, param
     id: ++self.id,
     jsonrpc: '2.0',
     method,
-    params
+    params: params && params.length
+      ? params
+      : null
   };
 }

packages/api-provider/src/types.d.ts

@@ -9,7 +9,7 @@ export type JsonRpcObject = {
 
 export type JsonRpcRequest = JsonRpcObject & {
   method: string;
-  params: Array<any>;
+  params: Array<any> | null;
 };
 
 export type JsonRpcResponseBase$Error = {