Fixing an issue that type 'null' and 'undefined' is not assignable to validateStatus when typescript strict option is enabled (#3200)

test/typescript/axios.ts

@@ -44,6 +44,14 @@ const config: AxiosRequestConfig = {
   cancelToken: new axios.CancelToken((cancel: Canceler) => {})
 };
 
+const nullValidateStatusConfig: AxiosRequestConfig = {
+  validateStatus: null
+};
+
+const undefinedValidateStatusConfig: AxiosRequestConfig = {
+  validateStatus: undefined
+};
+
 const handleResponse = (response: AxiosResponse) => {
   console.log(response.data);
   console.log(response.status);