FIX: Fixed 'OnDrop' event not called when 'IPointerDownHandler' is also listened. (ISXB-1014)

[bmalrat]

Description

When IPointerDownHandler is registered it enable the clic detector and after that PR #1911 it trigger all the time even if there is no pointerClickHandler.

var isClick = eventData.pointerClick == pointerClickHandler && eventData.eligibleForClick;
Is always true since pointerClickHandler and eventData.pointerClick are null.

Changes made

Added a null check to prevent a bad clic detection.
Also changed the code to prevent calling ExecuteEvents.GetEventHandler<IPointerClickHandler>(currentOverGo); twice

Testing

Tested with client project and some extra test with multiple MonoBehavior that use a combinaison of
IBeginDragHandler, IDragHandler, IDropHandler, IEndDragHandler, IPointerDownHandler, IPointerEnterHandler, IPointerExitHandler, IPointerClickHandler

Risk

Please describe the potential risks of your changes for the reviewers.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[ekcoh]

LGTM