Issue #2 - Return null instead of failing with a NPE

(cherry picked from commit 67038fc0d5ee87ed46f1076b7ec4506af331f2bf)

cpt/src/main/java/org/isisaddons/wicket/wickedcharts/cpt/ui/summarychart/CollectionContentsAsSummaryChart.java

@@ -160,15 +160,15 @@ private BigDecimal maxOf(BigDecimal max, final BigDecimal summaryMax) {
             ? summaryMax != null
                 ? max.max(summaryMax)
                 : max
-            : BigDecimal.ZERO;
+            : null;
     }
 
     private BigDecimal minOf(BigDecimal min, final BigDecimal summaryMin) {
         return min != null
             ? summaryMin != null
                 ? min.min(summaryMin)
                 : min
-            : BigDecimal.ZERO;
+            : null;
     }
 
     @Override