#### 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
statusText: 'NOT FOUND',
        responseText: 'Resource not found'
      });
      setTimeout(function () {
        expect(config.baseURL).toEqual('/api');
        expect(config.url).toEqual('/foo');
        done();
      }, 100);
    });
  });
  it('should allow overriding Content-Type header case-insensitive', function (done) {
    var response;
    var contentType = 'application/vnd.myapp.type+json';
    axios.post('/foo', { prop: 'value' }, {
      headers: {
        'content-type': contentType
      }
    }).then(function (res) {
      response = res;
    });
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders['Content-Type']).toEqual(contentType);
      done();
    });
  });
  it('should support binary data as array buffer', function (done) {
    var input = new Int8Array(2);
    input[0] = 1;
    input[1] = 2;
    axios.post('/foo', input.buffer);