<!-- 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
config[prop] = utils.deepMerge(config1[prop]);
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  utils.forEach(defaultToConfig2Keys, function defaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  var axiosKeys = valueFromConfig2Keys
    .concat(mergeDeepPropertiesKeys)
    .concat(defaultToConfig2Keys);
  var otherKeys = Object
    .keys(config2)
    .filter(function filterAxiosKeys(key) {
      return axiosKeys.indexOf(key) === -1;
    });
  utils.forEach(otherKeys, function otherKeysDefaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });
  return config;
};