Avoid division by 0 in eval script

scripts/conll18_ud_eval.py

@@ -456,8 +456,8 @@ def evaluate(gold_ud, system_ud, extraColumns) :
           goldValues.append(float(goldItem))
           predictedValues.append(float(systemItem))
 
-    R2 = None
-    if isNumericOnly :
+    R2 = 0.0
+    if isNumericOnly and len(goldValues) > 0 :
       correct[0] /= len(goldValues)
       correct[1] /= len(goldValues)
       goldMean = sum(goldValues) / len(goldValues)