#### 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
}).listen(4444, function () {
      var auth = { username: 'foo', password: 'bar' };
      var headers = { AuThOrIzAtIoN: 'Bearer 1234' }; /
      axios.get('http://localhost:4444/', { auth: auth, headers: headers }).then(function (res) {
        var base64 = Buffer.from('foo:bar', 'utf8').toString('base64');
        assert.equal(res.data, 'Basic ' + base64);
        done();
      });
    });
  });
  it('should provides a default User-Agent header', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers['user-agent']);
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        assert.ok(/^axios\/[\d.]+$/.test(res.data), `User-Agent header does not match: ${res.data}`);
        done();
      });
    });
  });
  it('should allow the User-Agent header to be overridden', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers['user-agent']);
    }).listen(4444, function () {
      var headers = { 'UsEr-AgEnT': 'foo bar' }; /
      axios.get('http://localhost:4444/', { headers }).then(function (res) {
        assert.equal(res.data, 'foo bar');
        done();
      });
    });