Check null when getting drop view in DragController

When the flag is on, there are some cases dropTarget is null:
TaplTestsTaskbar.testOpenMenu_fromTaskbarAllApps
TaplTestsTaskbar.testLaunchShortcutInSplitscreen
TaplTestsTaskbar.testLaunchShortcut

Add logic to check null before getting drop view and calling drop complete.

Bug: 343650193
Test: TaplTestsTaskbar.testOpenMenu_fromTaskbarAllApps
Test: TaplTestsTaskbar.testLaunchShortcutInSplitscreen
Test: TaplTestsTaskbar.testLaunchShortcut
Flag: com.android.launcher3.remove_apps_refresh_on_right_click
Change-Id: If8ce93bd6f7d0506e2272ca17dd7c19666549cfa