Plotting: Add protection for missing systematics (alisw#977)

Author: vkucera

machine_learning_hep/plotting/plot_jetsubstructure_run3.py

@@ -1230,7 +1230,8 @@ def plot(self):
                 1.0,
             )
             setup_legend(leg)
-            leg.AddEntry(list_syst_all[-1], " ", "FP")
+            if list_syst_all:
+                leg.AddEntry(list_syst_all[-1], " ", "FP")
             can.cd(2)
             leg.Draw()
             gStyle.SetErrorX(0.5)  # reset default width