REP: If length argument is None, let that stay so json does the right thing

We were converting it to a string with becomes 'None'
which then throws off the json parser

Author: waskyo

chb/buffer/LibraryCallCallsites.py

@@ -293,7 +293,10 @@ def to_json_result(self, app: 'AppAccess') -> JSONResult:
         content["buffersize"] = buffersize
         content["target-function"] = self.summary.name
         content["stack-offset"] = dstoffset
-        content["length-argument"] = str(self.lenarg)
+        if self.lenarg is not None:
+            content["length-argument"] = str(self.lenarg)
+        else:
+            content["length-argument"] = None
         content["spare"] = spare
 
         return JSONResult("librarycallsideeffect", content, "ok")