#### 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.

lib/adapters/http.js
options.hostname = proxy.host;
  options.host = proxy.host;
  options.port = proxy.port;
  options.path = location;
  
  if (proxy.auth) {
    var base64 = Buffer.from(proxy.auth.username + ':' + proxy.auth.password, 'utf8').toString('base64');
    options.headers['Proxy-Authorization'] = 'Basic ' + base64;
  }
  
  options.beforeRedirect = function beforeRedirect(redirection) {
    redirection.headers.host = redirection.host;
    setProxy(redirection, proxy, redirection.href);
  };
}
module.exports = function httpAdapter(config) {
  return new Promise(function dispatchHttpRequest(resolvePromise, rejectPromise) {
    var resolve = function resolve(value) {
      resolvePromise(value);
    };
    var reject = function reject(value) {
      rejectPromise(value);
    };
    var data = config.data;
    var headers = config.headers;
    
    
    
    if (!headers['User-Agent'] && !headers['user-agent']) {