Fix unhandled None in do_PDFStream, via oss-fuzz

pdfplumber/convert.py

@@ -109,8 +109,8 @@ class Serializer:
         else:
             return {k: self.serialize(v) for k, v in obj.items()}
 
-    def do_PDFStream(self, obj: Any) -> Dict[str, str]:
-        return {"rawdata": to_b64(obj.rawdata)}
+    def do_PDFStream(self, obj: Any) -> Dict[str, Optional[str]]:
+        return {"rawdata": to_b64(obj.rawdata) if obj.rawdata else None}
 
     def do_PSLiteral(self, obj: PSLiteral) -> str:
         return decode_text(obj.name)