[API Change] Initialise has optional cultureName but errors if not provided

[kkjamie]

public static bool Initialise(string path, string cultureName = null)

The 2nd parameter is optional.

Right now the documentation states, if we don't specify the cultureName it will fallback to the default culture. This behaviour is what happens although it does:
Debug.LogError("Invalid culture name: " + cultureName);
In this case culture name is empty so the error message is uninformative

This works perfectly if you provided an invalid one, but we should detect that we didn't actually provide one and make that acceptable behaviour, We could add a warning to indicate that one was not supplied and that we are falling back to a default.