JetUncertainties: Fix log(0)

A code quality check recently turned up a spot in JetUncertainties's plot-making utility where log(0) is explicitly taken. It's been this way since Athena was migrated from SVN, so I have no idea why it was like that to begin with and evidently nobody has ever actually tried to use it with the log-scale option. Nonetheless I'm fixing it here, replacing log(0) with 0 and removing a variable which was hard-coded to a constant 0 for some inscrutable reason. This is all used only for determining the location of a label on a plot.

This resolves ATLASSQ-417 and ATLASSQ-418.

Tagging @sschramm on the outside chance that he knows anything about this or has any comment.