[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/affix.js

@@ -21,8 +21,8 @@
       .on('click.bs.affix.data-api',  $.proxy(this.checkPositionWithEventLoop, this))
 
     this.$element     = $(element)
-    this.affixed      =
-    this.unpin        =
+    this.affixed      = null
+    this.unpin        = null
     this.pinnedOffset = null
 
     this.checkPosition()