Handle POST request without body in example client

When sending a POST request in the client example, we convert its
body to bytes to create the HMAC key. This fails when there is no
body because the ToBytes function does not handle the None value.
Return empty bytes in this case.

examples/example_client.py

@@ -257,6 +257,8 @@ def _CallHttpie( self, method, handler, data = None ):
 
 
 def ToBytes( value ):
+  if not value:
+    return bytes()
   if isinstance( value, bytes ):
     return value
   if isinstance( value, int ):