#### Describe the bug
The irrelevant error returned when requesting illegal URL, like: "tel:484-695-3408"
```
TypeError: Cannot read property 'replace' of null
    at dispatchHttpRequest (node_modules/axios/lib/adapters/http.js:109:74)
    at new Promise (<anonymous>)
    at httpAdapter (node_modules/axios/lib/adapters/http.js:46:10)
    at dispatchRequest (node_modules/axios/lib/core/dispatchRequest.js:52:10)
```

#### To Reproduce
```js
axios.get('tel:484-695-3408').catch(err => console.error(err));
```

#### Expected behavior
Return descriptive error, like "Unsupported protocol tel:"

#### Environment
 - Axios Version: 0.21.1
 - Adapter: HTTP
 - Browser: none
 - Browser Version: none
 - Node.js Version: 12.19.0
 - OS: MacOS 10.14
 - Additional Library Versions: none

#### Additional context/Screenshots
none
test/specs/requests.spec.js
axios('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      done();
    });
  });
  describe('timeouts', function(){
    beforeEach(function () {
      jasmine.clock().install();
    });
    afterEach(function () {
      jasmine.clock().uninstall();
    });
    it('should handle timeouts', function (done) {
      axios({
        url: '/foo',
        timeout: 100
      }).then(function () {
        fail(new Error('timeout error not caught'));
      }, function (err) {
        expect(err instanceof Error).toBe(true);
        expect(err.code).toEqual('ECONNABORTED');
        done();
      });
      jasmine.Ajax.requests.mostRecent().responseTimeout();
    });
    describe('transitional.clarifyTimeoutError', function () {
      it('should activate throwing ETIMEDOUT instead of ECONNABORTED on request timeouts', function (done) {
        axios({
          url: '/foo',