[Common] fixed computation of output quality for ReferenceComparatorC…

…heck

In the current code the output quality was always Null because
the initial Null value is always not better than any other quality
from the plots analysis.

Modules/Common/src/ReferenceComparatorCheck.cxx

@@ -210,8 +210,8 @@ Quality ReferenceComparatorCheck::check(std::map<std::string, std::shared_ptr<Mo
       continue;
     }
 
-    // update the overall quality
-    if (quality.isWorseThan(result)) {
+    // initialize or update the overall quality
+    if (result == Quality::Null || quality.isWorseThan(result)) {
       result.set(quality);
     }