Avoid empty file name for the user's report file

Avoid empty file name for the user's report file

If an empty user name creeps up to the name manufacturing function, the name
generated to store the user's files is empty and it leads to the deletion of
the whole report directory during the process. The visible results is that sarg
ends up with an error because its output directory is missing.

This patch makes sure no empty file name is used. It is still necessary to
avoid empty user names in the first place.