#### 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 proxy = config.proxy;
    if (!proxy && proxy !== false) {
      var proxyEnv = protocol.slice(0, -1) + '_proxy';
      var proxyUrl = process.env[proxyEnv] || process.env[proxyEnv.toUpperCase()];
      if (proxyUrl) {
        var parsedProxyUrl = url.parse(proxyUrl);
        var noProxyEnv = process.env.no_proxy || process.env.NO_PROXY;
        var shouldProxy = true;
        if (noProxyEnv) {
          var noProxy = noProxyEnv.split(',').map(function trim(s) {
            return s.trim();
          });
          shouldProxy = !noProxy.some(function proxyMatch(proxyElement) {
            if (!proxyElement) {
              return false;
            }
            if (proxyElement === '*') {
              return true;
            }
            if (proxyElement[0] === '.' &&
                parsed.hostname.substr(parsed.hostname.length - proxyElement.length) === proxyElement) {
              return true;
            }
            return parsed.hostname === proxyElement;
          });
        }
        if (shouldProxy) {
          proxy = {
            host: parsedProxyUrl.hostname,
            port: parsedProxyUrl.port,