bugfix for mobile phone support

on mobile phones only provide fallback image, if it's set on the
parallax-window element, otherwise don't

Author: wstoettinger

parallax.js

@@ -92,7 +92,7 @@
       this.positionY + (isNaN(this.positionY)? '' : 'px');
 
     if (navigator.userAgent.match(/(iPod|iPhone|iPad)/)) {
-      if (this.iosFix && !this.$element.is('img')) {
+      if (this.imageSrc && this.iosFix && !this.$element.is('img')) {
         this.$element.css({
           backgroundImage: 'url(' + this.imageSrc + ')',
           backgroundSize: 'cover',
@@ -103,7 +103,7 @@
     }
 
     if (navigator.userAgent.match(/(Android)/)) {
-      if (this.androidFix && !this.$element.is('img')) {
+      if (this.imageSrc && this.androidFix && !this.$element.is('img')) {
         this.$element.css({
           backgroundImage: 'url(' + this.imageSrc + ')',
           backgroundSize: 'cover',