Check assistant's password
I believe you're looking for String.IsNullOrEmpty.

// test for null or empty string
if (!String.IsNullOrEmpty(password) {
}

// -- OR --

// If the hasher call just can't accept null values, you can specify
// an empty string to fall back on like so:
(...LINQ...).FirstOrDefault(String.Empty);


I would also probably refactor this a bit:

public void CheckAssistantsPassword()
{
  using (DbClassesDataContext myDb = new DbClassesDataContext(dbPath))
  {
    var password = (from userAccounts in myDb.Assistants
                    where userAccounts.Ass_UserName == txtUserName.Text
                    select userAccounts.Ass_Password).FirstOrDefault(String.Empty);

    var hasher = new Hasher { SaltSize = 16 };
    if (hasher.CompareStringToHash(txtPassword.Text, password))
    {
      // Success
    }
    else
    {
      // Invalid password
    }
  }
}


Optionally you can place error checking on your context making sure you can establish a connection. Also, I recommend placing a using() block around your context to free it up after you're done with it.