Fix getStorageUsage() total_size value, when there are no Scenario objects in database

[mdukat]

When running frePPLe for the first time, clicking Help->About FrePPLe will throw Server Error 500:

Following error log in apache, we can see:

And that's, because there are no Scenario objects in database, and fetchone() returns () tuple, which we cannot select first element from.

My fix only add's size value from fetchone(), if there is anything to add. Tested on latest master branch build:

[jdetaeye]

An empty scenario table is not a normal situation. We always try to keep it in sync with the configured databases. But it doesn't hurt to have this case covered as well.

[mdukat]

Cool, thank you.