<!-- Click "Preview" for a more readable version --

Please read and follow the instructions before submitting an issue:

- Read all our documentation, especially the [README](https://github.com/axios/axios/blob/master/README.md). It may contain information that helps you solve your issue.
- Ensure your issue isn't already [reported](https://github.com/axios/axios/issues?utf8=%E2%9C%93&q=is%3Aissue).
- If you aren't sure that the issue is caused by Axios or you just need help, please use [Stack Overflow](https://stackoverflow.com/questions/tagged/axios) or [our chat](https://gitter.im/mzabriskie/axios).
- If you're reporting a bug, ensure it isn't already fixed in the latest Axios version.

⚠️👆 Feel free to these instructions before submitting the issue 👆⚠️
-->

**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/mergeConfig.js
* by merging two configuration objects together.
 *
 * @param {Object} config1
 * @param {Object} config2
 * @returns {Object} New object resulting from merging config2 to config1
 */
module.exports = function mergeConfig(config1, config2) {
  
  config2 = config2 || {};
  var config = {};
  var valueFromConfig2Keys = ['url', 'method', 'data'];
  var mergeDeepPropertiesKeys = ['headers', 'auth', 'proxy', 'params'];
  var defaultToConfig2Keys = [
    'baseURL', 'url', 'transformRequest', 'transformResponse', 'paramsSerializer',
    'timeout', 'withCredentials', 'adapter', 'responseType', 'xsrfCookieName',
    'xsrfHeaderName', 'onUploadProgress', 'onDownloadProgress',
    'maxContentLength', 'maxBodyLength', 'validateStatus', 'maxRedirects', 'httpAgent',
    'httpsAgent', 'cancelToken', 'socketPath', 'responseEncoding'
  ];
  utils.forEach(valueFromConfig2Keys, function valueFromConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    }
  });
  utils.forEach(mergeDeepPropertiesKeys, function mergeDeepProperties(prop) {
    if (utils.isObject(config2[prop])) {
      config[prop] = utils.deepMerge(config1[prop], config2[prop]);
    } else if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (utils.isObject(config1[prop])) {