#### 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
lib/adapters/http.js
var rejected = false;
    var reject = function reject(value) {
      done();
      rejected = true;
      rejectPromise(value);
    };
    var data = config.data;
    var headers = config.headers;
    var headerNames = {};
    Object.keys(headers).forEach(function storeLowerName(name) {
      headerNames[name.toLowerCase()] = name;
    });
    
    
    if ('user-agent' in headerNames) {
      
      if (!headers[headerNames['user-agent']]) {
        delete headers[headerNames['user-agent']];
      }
      
    } else {
      
      headers['User-Agent'] = 'axios/' + VERSION;
    }
    if (data && !utils.isStream(data)) {
      if (Buffer.isBuffer(data)) {
        
      } else if (utils.isArrayBuffer(data)) {
        data = Buffer.from(new Uint8Array(data));
      } else if (utils.isString(data)) {