🐛 [RUMF-1344] remove fallback for null scrollingElement

[bcaudan]

Motivation

Following #1688, current fallback does not improve much the situation.

Changes

• Remove fallback when scrolling element is null and don't handle document scroll in this case.
• Remove try/catch around weakmap as it should not happen anymore

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[BenoitZugmeyer]

🥜 nitpick: ‏I feel writing it like that would be a tiny bit clearer, but either is fine

      if (element === document) {
        if (!document.scrollingElement) {
          // cf https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement,
          // in some cases scrolling elements can not be defined, we don't support those for now
          return
        }
        element = document.scrollingElement
      }
      scrollPositionsByElement.set((element as Element), scrollPositions)