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**Describe the bug**

Param with value of null is replaced with "{}"

**To Reproduce**

axios.get('/whatever', {params: {test: null}}) used to make GET request to /whatever, now it's /whatever?test=%7B%7D

**Expected behavior**

Should request to /whatever.

**Environment:**
 - Axios Version [the current master]

**Additional context/Screenshots**

See https://github.com/axios/axios/pull/2656#issuecomment-598269460.
lib/core/dispatchRequest.js
'use strict';
var utils = require('./../utils');
var transformData = require('./transformData');
var isCancel = require('../cancel/isCancel');
var defaults = require('../defaults');
/**
 * Throws a `Cancel` if cancellation has been requested.
 */
function throwIfCancellationRequested(config) {
  if (config.cancelToken) {
    config.cancelToken.throwIfRequested();
  }
}