Fix NPE in SyncReviewerManager

A NullPointerException occurred when attempting to auto-assign reviewers
synchronously, without explicitly registering the AutoassignModule in
gerrit.config. This prevented reviewers from being assigned to changes.

This was caused by a mismerge of 3.4 (Iff9b12), which needlessly kept
calling `addToAttentionSet` even when the full list of reviewers had
already been computed.

Bug: Issue 353080249
Change-Id: Idbeb9c5ce2235e4b97f2ca4daa18f0f8e234093f

owners-autoassign/src/main/java/com/googlesource/gerrit/owners/common/SyncReviewerManager.java

diff --git a/owners-autoassign/src/main/java/com/googlesource/gerrit/owners/common/SyncReviewerManager.java b/owners-autoassign/src/main/java/com/googlesource/gerrit/owners/common/SyncReviewerManager.java
index bdfa7f6..3650e4f 100644
--- a/owners-autoassign/src/main/java/com/googlesource/gerrit/owners/common/SyncReviewerManager.java
+++ b/owners-autoassign/src/main/java/com/googlesource/gerrit/owners/common/SyncReviewerManager.java
@@ -141,7 +141,7 @@
 
         in.ignoreAutomaticAttentionSetRules = true;
         in.addToAttentionSet =
-            ownersForAttentionSet.get().addToAttentionSet(changeInfo, reviewersAccounts).stream()
+            reviewersAccounts.stream()
                 .map(
                     (reviewer) ->
                         new AttentionSetInput(