Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant)

Qt-only changes.

src/qt/bantablemodel.cpp

@@ -76,7 +76,7 @@ public:
         if (idx >= 0 && idx < cachedBanlist.size())
             return &cachedBanlist[idx];
 
-        return 0;
+        return nullptr;
     }
 };