cache-tree.c::cache_tree_find(): simplify internal API

Earlier cache_tree_find() needs to be called with a valid cache_tree,
but repeated look-up may find an invalid or missing cache_tree in between.
Help simplify the callers by returning NULL to mean "nothing appropriate
found" when the input is NULL.

Signed-off-by: Junio C Hamano <gitster@pobox.com>