[v6r11] Fix the case when some files only have a local replica (they were considered failed)

This is relatively urgent as user jobs may not be able to run...
This pattern is not correct although it was there even before. The correct one must be:

ret = S_OK( {'Successful': [], 'Failed': []} )

or

ret['Value'] = {'Successful': [], 'Failed': []}

According to the DIRAC standard return structure the Successful/Failed dictionary for bulk queries must be returned in the 'Value' key of the dictionary. In this particular case, the returned value is not checked for the Successful/Failed details, that  is why it was not noticed before.