[Ref #15881] Use Explicit Values rather than Chain

[Ref #15881] Use Explicit Values for javascript variables rather than chained ones.

From https://github.com/twbs/bootstrap/pull/15881#discussion_r25330647

js/tooltip.js

@@ -15,11 +15,11 @@
   // ===============================
 
   var Tooltip = function (element, options) {
-    this.type       =
-    this.options    =
-    this.enabled    =
-    this.timeout    =
-    this.hoverState =
+    this.type       = null
+    this.options    = null
+    this.enabled    = null
+    this.timeout    = null
+    this.hoverState = null
     this.$element   = null
 
     this.init('tooltip', element, options)