[Sync::USS] FIX DCHECK crash due missing parent id in uss_migrator.cc

This CL checks bookmark fields for validity/existence before parsing
them when migrating from directory to USS architectures
in uss_migrator.cc.

The original problematic code has been introduced in:
https://chromium-review.googlesource.com/c/chromium/src/+/1162226

Bug: 516866,871671
Change-Id: I843c62325f3e3ec37ce05783c999bdfd5f3542a0
Reviewed-on: https://chromium-review.googlesource.com/1164946
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581186}

components/sync/engine_impl/uss_migrator.cc

@@ -44,9 +44,13 @@ bool ExtractSyncEntity(ReadTransaction* trans,
   entity->set_client_defined_unique_tag(entry.GetUniqueClientTag());
   // Required fields for bookmarks only.
   entity->set_folder(entry.GetServerIsDir());
-  entity->set_parent_id_string(entry.GetServerParentId().GetServerId());
-  *entity->mutable_unique_position() =
-      entry.GetServerUniquePosition().ToProto();
+  if (!entry.GetServerParentId().IsNull()) {
+    entity->set_parent_id_string(entry.GetServerParentId().GetServerId());
+  }
+  if (entry.GetServerUniquePosition().IsValid()) {
+    *entity->mutable_unique_position() =
+        entry.GetServerUniquePosition().ToProto();
+  }
   entity->set_server_defined_unique_tag(entry.GetUniqueServerTag());
 
   // It looks like there are fancy other ways to get e.g. passwords specifics