Return None in case of no contacts in QS-score

The score is simply not valid and returning 0.0 as done before is incorrect.
Delegates further treatment of the problem to the caller which might want to
say: if both structures are monomer, I still give it a value of 1. Or, to
extend that definition, if I reproduced the correct stoichiometry, I still
give it a value of 1.

modules/mol/alg/pymod/qsscore.py

@@ -255,6 +255,8 @@ class QSScorerResult:
     def QS_best(self):
         """ QS_best - the actual score as described in formula section above
 
+        Returns None if there are no contacts in the compared structures
+
         :type: :class:`float`
         """
         nominator = self.weighted_scores
@@ -262,12 +264,14 @@ class QSScorerResult:
         if denominator != 0.0:
             return nominator/denominator
         else:
-            return 0.0
+            return None
 
     @property
     def QS_global(self):
         """ QS_global - the actual score as described in formula section above
 
+        Returns None if there are no contacts in the compared structures
+
         :type: :class:`float`
         """
         nominator = self.weighted_scores
@@ -275,7 +279,7 @@ class QSScorerResult:
         if denominator != 0.0:
             return nominator/denominator
         else:
-            return 0.0
+            return None
 
 
 class QSScorer: