[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/carousel.js

@@ -17,10 +17,10 @@
     this.$element    = $(element)
     this.$indicators = this.$element.find('.carousel-indicators')
     this.options     = options
-    this.paused      =
-    this.sliding     =
-    this.interval    =
-    this.$active     =
+    this.paused      = null
+    this.sliding     = null
+    this.interval    = null
+    this.$active     = null
     this.$items      = null
 
     this.options.keyboard && this.$element.on('keydown.bs.carousel', $.proxy(this.keydown, this))