rgw: Avoid dereferencing nullptr while configuring bucket sync policy

While configuring bucket sync policy, in "rgw_sync_bucket_entities::set_bucket()",
there could be a case where in bucket doesnt contain any value but is still being
dereferenced. This commit fixes the same.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>

src/rgw/rgw_sync_policy.cc

@@ -258,6 +258,10 @@ void rgw_sync_bucket_entities::set_bucket(std::optional<string> tenant,
     bucket.emplace();
   }
 
+  if (!bucket) {
+    return;
+  }
+
   set_bucket_field(tenant, &bucket->tenant);
   set_bucket_field(bucket_name, &bucket->name);
   set_bucket_field(bucket_id, &bucket->bucket_id);