create a new empty database when deleting the old one

when --delete is used we should leave an empty database in place. If
fail2ban_digest is used via cron the missing DB will cause an error to
be triggered and the admin will be mailed because one cron job failed,
which is not the case actually

bin/fail2ban_digest

@@ -116,14 +116,19 @@ def add(db, ip):
 
 def digest(db, delete):
 	db_file = db_location + '/' + db + '.dbm'
+	new_db_file = db_location + '/.' + db + '.dbm'
 	try:
 		db = db_busy_open(db_file, 'r', 30)  # this is just a trick to lock the DB, to have a consistent copy
 	except OSError as e:
 		# we raise a RuntimeError instead of an OSError to have prettier output
 		raise RuntimeError('Error while opening database file %s: %s' % (repr(db_file), str(e))) from None
 	if delete:
-		#os.rename(db_file, db_file + '.digest') # just in case we want to delay the removal in the future
-		os.unlink(db_file)
+		# wipe the DB, leave an empty one so maildigest will find it (if called from cron a failure would mail
+		# the admin)
+		with dbm.open(new_db_file, 'n') as new_db:
+			new_db[db_creation_date_key] = datetime.utcnow().strftime(db_date_format).encode('UTF-8')
+		os.rename(new_db_file, db_file)
+
 	try:
 		db_creation_date = db[db_creation_date_key].decode('UTF-8')
 	except KeyError as e: