defining the proper variable for PayerNameNVP in DoNonReferencedCredit

In `PayPal::DoNonReferencedCredit()`, the `$PayerName`  variable is wrongly defined - `$PayerNameNVP` must be defined instead, as it is used in the method. 

Using the current version of the code throws a notice for usage of undefined `$PayerNameNVP`. Replacing the `$PayerName = ''` definition with `$PayerNameNVP = ''` fixes this issue.