Update lowerColor to optional

Author: Adrian Wu

src/elements/boxandwhiskers.js

@@ -106,11 +106,13 @@ const BoxAndWiskers = Chart.elements.BoxAndWhiskers = ArrayElementBase.extend({
     ctx.lineTo(x0 + width, boxplot.median);
 
     // fill the part below the median with lowerColor
-    ctx.fillStyle = vm.lowerColor;
-    if (boxplot.q3 > boxplot.q1) {
-      ctx.fillRect(x0, boxplot.median, width, boxplot.q3 - boxplot.median);
-    } else {
-      ctx.fillRect(x0, boxplot.median, width, boxplot.q1 - boxplot.median);
+    if (vm.lowerColor) {
+      ctx.fillStyle = vm.lowerColor;
+      if (boxplot.q3 > boxplot.q1) {
+        ctx.fillRect(x0, boxplot.median, width, boxplot.q3 - boxplot.median);
+      } else {
+        ctx.fillRect(x0, boxplot.median, width, boxplot.q1 - boxplot.median);
+      }
     }
 
     ctx.closePath();