default out-marshaller for ForeignPtr adds finalizer for null ptr

Sometimes C functions return null pointers. The default marshaller for ForeignPtr's in c2hs still adds a finalizer for null pointers, which causes the application to crash when the memory is freed. I suggest the default marshaller should check if the ptr equals `nullPtr` and, if so, use `newForeignPtr_` instead.

Locate c2hs's default out-marshaller for ForeignPtr and review how it handles a null pointer returned by a C function. Reproduce the null-pointer case and verify that the marshaller avoids installing a finalizer for nullPtr while retaining normal finalizer behavior for non-null pointers.

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