Fix stdin handling for file-scope

Regardless of input type, we should use default handling if we are
dealing with stdin. In other words, there should be no file-scope if
there are no files. This was an issue with pandoc json, which could be
piped on stdin, but which was read by default with `--file-scope`.

pandoc.hs

@@ -1327,6 +1327,7 @@ convertWithOpts opts args = do
   (doc, media) <- case reader of
     (StringReader _) | not fileScope && readerName' /= "json" ->
                          sourceToDoc sources
+    _ | null sources -> sourceToDoc sources
     _  -> do pairs <- mapM (\s -> sourceToDoc [s]) sources
              return (mconcat $ map fst pairs, mconcat $ map snd pairs)