<!-- 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.
test/specs/utils/isX.spec.js
var utils = require('../../../lib/utils');
var Stream = require('stream');
describe('utils::isX', function () {
  it('should validate Array', function () {
    expect(utils.isArray([])).toEqual(true);
    expect(utils.isArray({length: 5})).toEqual(false);
  });
  it('should validate Buffer', function () {
    expect(utils.isBuffer(Buffer.from('a'))).toEqual(true);
    expect(utils.isBuffer(null)).toEqual(false);
    expect(utils.isBuffer(undefined)).toEqual(false);
  });
  it('should validate ArrayBuffer', function () {
    expect(utils.isArrayBuffer(new ArrayBuffer(2))).toEqual(true);
    expect(utils.isArrayBuffer({})).toEqual(false);
  });
  it('should validate ArrayBufferView', function () {
    expect(utils.isArrayBufferView(new DataView(new ArrayBuffer(2)))).toEqual(true);
  });
  it('should validate FormData', function () {
    expect(utils.isFormData(new FormData())).toEqual(true);
  });
  it('should validate Blob', function () {
    expect(utils.isBlob(new Blob())).toEqual(true);
  });
  it('should validate String', function () {
    expect(utils.isString('')).toEqual(true);
    expect(utils.isString({toString: function () { return ''; }})).toEqual(false);
  });