Add Tests on scrollbar.js & better handling if a style property doesn't exists (#33948)

* scrollbar.js:
add some tests
transfer test from modal.spec. to scrollbar.spec
proper handling if style property doesn't exist

js/src/util/scrollbar.js

@@ -44,8 +44,11 @@ const _setElementAttributes = (selector, styleProp, callback) => {
       }
 
       const actualValue = element.style[styleProp]
+      if (actualValue) {
+        Manipulator.setDataAttribute(element, styleProp, actualValue)
+      }
+
       const calculatedValue = window.getComputedStyle(element)[styleProp]
-      Manipulator.setDataAttribute(element, styleProp, actualValue)
       element.style[styleProp] = `${callback(Number.parseFloat(calculatedValue))}px`
     })
 }