A NativeContext is initialized in two steps: First the map is allocated,
only afterwards the NativeContext. It could happen that there is a GC
happening when allocating the NativeContext. In such a case the
native_context for the Map is still set to null.

Fix this by also allowing null in Map::MapVerify.

Bug: v8:11695
Change-Id: Id8dcd6aef83aff4cbfff45a1e993e555cff8e7bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2853587Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74237}