Update c3.js and c3.min.js

c3.js

@@ -3627,8 +3627,9 @@
                 text = "<table class='" + CLASS.tooltip + "'>" + (title || title === 0 ? "<tr><th colspan='2'>" + title + "</th></tr>" : "");
             }
 
-            name = nameFormat(d[i].name, d[i].ratio, d[i].id, d[i].index);
             value = valueFormat(d[i].value, d[i].ratio, d[i].id, d[i].index);
+            if (value !== undefined) {
+                name = nameFormat(d[i].name, d[i].ratio, d[i].id, d[i].index);
                 bgcolor = $$.levelColor ? $$.levelColor(d[i].value) : color(d[i].id);
 
                 text += "<tr class='" + CLASS.tooltipName + "-" + d[i].id + "'>";
@@ -3636,6 +3637,7 @@
                 text += "<td class='value'>" + value + "</td>";
                 text += "</tr>";
             }
+        }
         return text + "</table>";
     };
     c3_chart_internal_fn.tooltipPosition = function (dataToShow, tWidth, tHeight, element) {