Merge pull request #39333 from cyx1231st/wip-seastore-onode-tree-empty-oid-ns

crimson/onode-staged-tree: support empty ns and oid

Reviewed-by: Samuel Just <sjust@redhat.com>

diff --cc src/crimson/os/seastore/onode_manager/staged-fltree/stages/key_layout.h
Simple merge

diff --cc src/crimson/os/seastore/onode_manager/staged-fltree/tree_utils.h
Simple merge