add proper support for tm_user_webdir = None

[belforte]

current code fails with HTTP 500.
Returning "None" is a gentler way which allows the calling client to deal with it.
Ref: #6544 (comment)

code is

CRABServer/src/python/CRABInterface/RESTTask.py

Lines 218 to 224 in 483adb2

	if row.user_webdir:
	#extract /cms1425/taskname from the user webdir
	suffix = re.search(r"(/[^/]+/[^/]+/?)$", row.user_webdir).group(0)
	else:
	raise ExecutionError("Webdir not set in the database. Cannot build proxied webdir")

And in the example reported in there the DB has this (from the UI )

[belforte]

closed via #8767