fix: fixed issues handling empty keys + separator characters in the key

[raing3]

PR is to fix a couple of issues I encountered while trying to use this library:

1. If you have an empty key in your JS object it disappears from the object when sorting it.
2. If your key contains the separator string it disappears from the object when sorting it.
3. If a "null" value is encountered in the encoded JSON an error was previously thrown (null is an object so passes through the existing if logic but then Object.keys throws).

Notes:

• Hopefully the tests show the issues well enough.
• Haven't had to handle interpreting escape sequences before so there may be a better way to do this, or may be something I have overlooked but tried to make the tests cover as many different odd scenarios I could think of.
• Could be argued to be a BC breaking change as the "" character is no longer supported.

Let me know if any changes are needed to get this merged and released. Thanks!