Return null if obj is of type PdfNull

fixes ststeiger#287

Author: dampee

PdfSharpCore/Pdf/PdfDictionary.cs

@@ -395,6 +395,10 @@ public int GetInteger(string key, bool create)
                         this[key] = new PdfInteger();
                     return 0;
                 }
+
+                if (obj is PdfNull)
+                    return 0;
+
                 PdfReference reference = obj as PdfReference;
                 if (reference != null)
                     obj = reference.Value;