#### 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
response = res;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        statusText: 'OK',
        responseText: '{"foo": "bar"}',
        headers: {
          'Content-Type': 'application/json'
        }
      });
      setTimeout(function () {
        expect(response.data.foo).toEqual('bar');
        expect(response.status).toEqual(200);
        expect(response.statusText).toEqual('OK');
        expect(response.headers['content-type']).toEqual('application/json');
        done();
      }, 100);
    });
  });
  it('should not modify the config url with relative baseURL', function (done) {
    var config;
    axios.get('/foo', {
        baseURL: '/api'
    }).catch(function (error) {
        config = error.config;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 404,