Parsing of `auth` differs from Node's behaviour

## Expected Behavior

When the username is empty and a password is available, the returned value of `auth` should align with output from Node's `url` module.

```js
const url = require('url');
const nativeUrl = require('native-url');

url.parse('http://:password@test.com');
// { auth: ':password', ... }

nativeUrl.parse('http://:password@test.com');
// { auth: ':password', ... }
```

## Actual Behavior

```js
const url = require('url');
const nativeUrl = require('native-url');

url.parse('http://:password@test.com');
// { auth: ':password', ... }

nativeUrl.parse('http://:password@test.com');
// { auth: 'password', ... }
```

## Steps to Reproduce the Problem

As described in code blocks above.

The culprit is this block, where all falsy values are filtered out.

https://github.com/GoogleChromeLabs/native-url/blob/0c5aec20a765bc6a396b6066abf6951dbb5d000d/src/parse.js#L188-L191

I do think that this should technically be non-compliant with the spec (HTTP Basic Auth requires username to be defined) though ... ?

## Specifications

- Version: 0.2.6
- Platform: macOS 10.15.5