#### 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/unit/adapters/http.js
});
      });
    });
  });
  it('should support cancel', function (done) {
    var source = axios.CancelToken.source();
    server = http.createServer(function (req, res) {
      // c
      source.cancel('Operation has been canceled.');
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        cancelToken: source.token
      }).catch(function (thrown) {
        assert.ok(thrown instanceof axios.Cancel, 'Promise must be rejected with a Cancel object');
        assert.equal(thrown.message, 'Operation has been canceled.');
        done();
      });
    });
  });
  it('should combine baseURL and url', function (done) {
    server = http.createServer(function (req, res) {
      res.end();
    }).listen(4444, function () {
      axios.get('/foo', {
        baseURL: 'http://localhost:4444/',
      }).then(function (res) {
        assert.equal(res.config.baseURL, 'http://localhost:4444/');
        assert.equal(res.config.url, '/foo');
        done();
      });