BUG: Avoid str concat with None when logging user_intent

[serhatgktp]

The following two lines occasionally crash the program:

user_intent = get_first_nonempty_line(result)
log.info("Canonical form for user intent: " + user_intent)

because user_intent may be equal to None. This is also seemingly expected as the very next line of code checks for it.

if user_intent is None:
    ...

Thus it should make sense to alter the logging line to prevent the program from crashing, as follows:

log.info("Canonical form for user intent: " + (user_intent if user_intent else "None"))