BKK: fix formating error in logging
Fixes
Traceback (most recent call last):
File "/opt/dirac/versions/v9r4p11_1578405262/LHCbDIRAC/BookkeepingSystem/Service/BookkeepingManagerHandler.py", line 111, in export_sendXMLBookkeepingReport
retVal = reader_.readXMLfromString(xml)
File "/opt/dirac/versions/v9r4p11_1578405262/LHCbDIRAC/BookkeepingSystem/Service/XMLReader/XMLFilesReaderManager.py", line 93, in readXMLfromString
result = self.processJob(job)
File "/opt/dirac/versions/v9r4p11_1578405262/LHCbDIRAC/BookkeepingSystem/Service/XMLReader/XMLFilesReaderManager.py", line 440, in processJob
gLogger.debug('The visibility flag is:' + outputFileTypes[ftype])
TypeError: cannot concatenate 'str' and 'NoneType' objects