TotalDebt sometimes returns `null`

[Matthiee]

At the moment the field for totalDebt is not nullable. https://github.com/MatthiWare/FinancialModelingPrep.NET/blob/5a9129b9c40ef7b418853553bebf4fbae26e7bb7/FinancialModelingPrepApi/Model/CompanyValuation/BalanceSheetResponse.cs#L158

Sometimes FMP API returns null for this field.

I created a support ticket to check if this is valid or not.