fix pdf_to_img

@@ -205,7 +205,11 @@ class PDFHandler:
             tpix = fitz.Pixmap(fitz.csRGB, pix)
             return tpix
         else:
-            return doc.extractImage(xref)
+            res = doc.extractImage(xref)
+            if res is None:
+                return fitz.Pixmap(doc, xref)
+            else:
+                return res
 
     @staticmethod
     def get_img_data(pix):