Fix python issue with ambiguous boolean

tools/plotting/fastnnlo_multigridclosure.py

@@ -387,9 +387,9 @@ def main():
 
     # Conversion of NNLOJET fb (15) to publication units
     fbtoxsunit = pow(10,xsunit-15)
-    if xs_nnlo:
+    if xs_nnlo is not None:
         xs_nnlo = xs_nnlo*fbtoxsunit
-    if xs_nnlo_list:
+    if xs_nnlo_list is not None:
         for i in range(len(xs_nnlo_list)):
             xs_nnlo_list[i] = xs_nnlo_list[i]*fbtoxsunit