Changed the 'pipe' utility to work with commands taking input

on standard in, also when providing an emtpy string as input.
This is now different from providing 'None' as the input
argument, or omitting it.
 - Legacy-Id: 6485

ietf/utils/pipe.py

@@ -9,7 +9,7 @@ def pipe(cmd, str=None):
         bufsize = len(str)
 
     pipe = Popen(cmd, True, bufsize)
-    if str:
+    if not str is None:
         pipe.tochild.write(str)
         pipe.tochild.close()