Allow passwords of zero length

[andreimilto]

Currently a password of zero length causes ArgumentException in the constructor:

public Argon2(byte[] password)
{
    if (password == null || password.Length == 0)
        throw new ArgumentException("Argon2 needs a password set", nameof(password));

    _password = password;
}

However, using a password of zero length as an input for Argon2 is an accepted practice:

• specification v1.3 allows it explicitly (section 3.1);
• another Argon2 library for .NET supports such passwords.

So, it would be nice to see the support of zero-length passwords here.