Fix Python syntax error in api_server roc_curve.py

Discovered in kubeflow#1721

$ __python2 -c "'a' is not in 'abc'"__  # --> SyntaxError: invalid syntax

Your reviews please @Ark-kun @ajchili 

/assign @neuromage

backend/src/apiserver/visualization/roc_curve.py

@@ -34,7 +34,7 @@
 # trueclass
 # true_score_column
 
-if "is_generated" is not in variables or variables["is_generated"] is False:
+if variables.get("is_generated", False) is False:
     # Create data from specified csv file(s).
     # The schema file provides column names for the csv file that will be used
     # to generate the roc curve.