#### 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
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,
            protocol: parsedProxyUrl.protocol
          };
          if (parsedProxyUrl.auth) {
            var proxyUrlAuth = parsedProxyUrl.auth.split(':');
            proxy.auth = {
              username: proxyUrlAuth[0],