Increase lazy loader performance

This stops the checkElementsInView method from running when there aren't any lazy load images on the page. By calling this method we are causing an unnecessary reflow which on large pages is a massive performance issue

app/assets/javascripts/lazy_loader.js

-/* eslint-disable one-export, one-var, one-var-declaration-per-line */
-
 import _ from 'underscore';
 
 export const placeholderImage = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
@@ -21,7 +19,10 @@ export default class LazyLoader {
   }
   searchLazyImages() {
     this.lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
-    this.checkElementsInView();
+
+    if (this.lazyImages.length) {
+      this.checkElementsInView();
+    }
   }
   startContentObserver() {
     const contentNode = document.querySelector(this.observerNode) || document.querySelector('body');
@@ -45,15 +46,13 @@ export default class LazyLoader {
   checkElementsInView() {
     const scrollTop = pageYOffset;
     const visHeight = scrollTop + innerHeight + SCROLL_THRESHOLD;
-    let imgBoundRect, imgTop, imgBound;
 
     // Loading Images which are in the current viewport or close to them
     this.lazyImages = this.lazyImages.filter((selectedImage) => {
       if (selectedImage.getAttribute('data-src')) {
-        imgBoundRect = selectedImage.getBoundingClientRect();
-
-        imgTop = scrollTop + imgBoundRect.top;
-        imgBound = imgTop + imgBoundRect.height;
+        const imgBoundRect = selectedImage.getBoundingClientRect();
+        const imgTop = scrollTop + imgBoundRect.top;
+        const imgBound = imgTop + imgBoundRect.height;
 
         if (scrollTop < imgBound && visHeight > imgTop) {
           LazyLoader.loadImage(selectedImage);