Check_MK Werk 3371: Be compatible to old site=None setting read from hosts.mk

ID: 3371 Title: Be compatible to old site=None setting read from hosts.mk Component: WATO Level: 1 Class: Bug Fix Version: 1.2.9i1 Old WATO versions saved the site attribute with a value of None which is not allowed in current WATO versions. Either the key has to be missing when no specific site is configured or the key has to be missing. Cleaned this up. Fixed exception: TypeError (cannot concatenate 'str' and 'NoneType' objects)