fixes errors on null data

fixes errors thrown when provided with null array e.g.
['data',null,null]

c3.js

@@ -1250,6 +1250,12 @@
             if (yTargets.length === 0) { // use current domain if target of axisId is none
                 return axisId === 'y2' ? y2.domain() : y.domain();
             }
+            if (isNaN(yDomainMin)) { // set minimum to zero when not number
+                yDomainMin = 0;
+            }
+            if (isNaN(yDomainMax)) { // set maximum to have same value as yDomainMin
+                yDomainMax = yDomainMin;
+            }
             if (yDomainMin === yDomainMax) {
                 yDomainMin < 0 ? yDomainMax = 0 : yDomainMin = 0;
             }