#### Is your feature request related to a problem? Please describe.
Some APIs only work if the User-Agent header is entirely omitted.  This is not currently possible in Axios in NodeJS.  #1411 opened this discussion some time back, but was closed without an actual solution other than commenting out the code which inserts the header.

#### Describe the solution you'd like
There should be a way to completely omit the U-A header if desired.  User-Agent is not a required header, and its deprecation (in favor of Client-Hints) is being actively discussed.

The simplest solution would probably to retain the current behavior if no User-Agent header is specified, but omit the header if it is explicitly provided with a falsy value (`null` and `""` both seem to clearly communicate the caller's intent not to have a value here).

#### Describe alternatives you've considered
Explicit options to disable the current behavior could work, but that seems like a less-elegant path.

#### Additional context
No.

test/unit/adapters/http.js
});
    });
  });
  it('should support HTTP proxies', function (done) {
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end('12345');
    }).listen(4444, function () {
      proxy = http.createServer(function (request, response) {
        var parsed = url.parse(request.url);
        var opts = {
          host: parsed.hostname,
          port: parsed.port,
          path: parsed.path
        };
        http.get(opts, function (res) {
          var body = '';
          res.on('data', function (data) {
            body += data;
          });
          res.on('end', function () {
            response.setHeader('Content-Type', 'text/html; charset=UTF-8');
            response.end(body + '6789');
          });
        });
      }).listen(4000, function () {
        axios.get('http://localhost:4444/', {
          proxy: {
            host: 'localhost',
            port: 4000