#### 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 supply a user-agent if one is not specified', function (done) {
    server = http.createServer(function (req, res) {
      assert.equal(req.headers["user-agent"], 'axios/' + pkg.version);
      res.end();
    }).listen(4444, function () {
      axios.get('http://localhost:4444/'
      ).then(function (res) {
        done();
      });
    });
  });
  it('should omit a user-agent if one is explicitly disclaimed', function (done) {
    server = http.createServer(function (req, res) {
      assert.equal("user-agent" in req.headers, false);
      assert.equal("User-Agent" in req.headers, false);
      res.end();
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        headers: {
          "User-Agent": null
        }
      }
      ).then(function (res) {
        done();
      });
    });
  });
  it('should throw an error if http server that aborts a chunked request', function (done) {