Update c3.js and c3.min.js

c3.js

@@ -3627,14 +3627,16 @@
                 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);
-            bgcolor = $$.levelColor ? $$.levelColor(d[i].value) : color(d[i].id);
-
-            text += "<tr class='" + CLASS.tooltipName + "-" + d[i].id + "'>";
-            text += "<td class='name'><span style='background-color:" + bgcolor + "'></span>" + name + "</td>";
-            text += "<td class='value'>" + value + "</td>";
-            text += "</tr>";
+            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 + "'>";
+                text += "<td class='name'><span style='background-color:" + bgcolor + "'></span>" + name + "</td>";
+                text += "<td class='value'>" + value + "</td>";
+                text += "</tr>";
+            }
         }
         return text + "</table>";
     };