#### 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
password: proxyUrlAuth[1]
            };
          }
        }
      }
    }
    if (proxy) {
      options.headers.host = parsed.hostname + (parsed.port ? ':' + parsed.port : '');
      setProxy(options, proxy, protocol + '//' + parsed.hostname + (parsed.port ? ':' + parsed.port : '') + options.path);
    }
    var transport;
    var isHttpsProxy = isHttpsRequest && (proxy ? isHttps.test(proxy.protocol) : true);
    if (config.transport) {
      transport = config.transport;
    } else if (config.maxRedirects === 0) {
      transport = isHttpsProxy ? https : http;
    } else {
      if (config.maxRedirects) {
        options.maxRedirects = config.maxRedirects;
      }
      transport = isHttpsProxy ? httpsFollow : httpFollow;
    }
    if (config.maxBodyLength > -1) {
      options.maxBodyLength = config.maxBodyLength;
    }
    
    var req = transport.request(options, function handleResponse(res) {
      if (req.aborted) return;
      
      var stream = res;