#### 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
var axios = require('../../../index');
var http = require('http');
var https = require('https');
var net = require('net');
var url = require('url');
var zlib = require('zlib');
var assert = require('assert');
var fs = require('fs');
var path = require('path');
var server, proxy;
describe('supports http with nodejs', function () {
  afterEach(function () {
    if (server) {
      server.close();
      server = null;
    }
    if (proxy) {