#### 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
assert.equal(error.message, 'timeout of 250ms exceeded');
        done();
      }, 300);
    });
  });
  it('should allow passing JSON', function (done) {
    var data = {
      firstName: 'Fred',
      lastName: 'Flintstone',
      emailAddr: 'fred@example.com'
    };
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'application/json;charset=utf-8');
      res.end(JSON.stringify(data));
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        assert.deepEqual(res.data, data);
        done();
      });
    });
  });
  it('should allow passing JSON with BOM', function (done) {
    var data = {
      firstName: 'Fred',
      lastName: 'Flintstone',
      emailAddr: 'fred@example.com'
    };
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'application/json;charset=utf-8');
      var bomBuffer = Buffer.from([0xEF, 0xBB, 0xBF])