Avoid triggering an exception when shutting down the line editor

This commit avoids triggering an internal exception when shutting down
the line editor which can lead to data loss on exit, especially on
Windows. Thanks go to GitHub user jerrbe for reporting this issue.

asyncssh/editor.py

@@ -704,8 +704,10 @@ def process_input(self, data: str, datatype: DataType) -> None:
                     self._ring_bell()
 
             self._bell_rung = False
-            self._chan.write(''.join(self._outbuf))
-            self._outbuf.clear()
+
+            if self._outbuf:
+                self._chan.write(''.join(self._outbuf))
+                self._outbuf.clear()
         else:
             self._session.data_received(data, datatype)