Remove redundant sanity checks

unnecessary, as these checks should never fail (since the handler would not be called if there weren't any tabs, and the `index > 0` clause in the preceding `if` ensures that `index` will never be negative

js/src/tab.js

@@ -245,10 +245,6 @@ class Tab {
 
     const tabs = makeArray(SelectorEngine.find(Selector.DATA_TOGGLE, tablist))
 
-    if (!tabs.length) {
-      return
-    }
-
     let index = tabs.indexOf(event.target)
     const tabInstance = Data.getData(tabs[index], DATA_KEY) || new Tab(tabs[index])
 
@@ -266,10 +262,6 @@ class Tab {
       index++
     }
 
-    if (index < 0) {
-      index = 0
-    }
-
     tabs[index].focus()
     tabs[index].click()
   }