split-index: accept that a base index can be empty
We are about to fix an ancient bug where `do_read_index()` pretended that the index was not initialized when there are no index entries. Before the `index_state` structure gained the `initialized` flag in 913e0e99b6a (unpack_trees(): protect the handcrafted in-core index from read_cache(), 2008-08-23), that was the best we could do (even if it was incorrect: it is totally possible to read a Git index file that contains no index entries). This pattern was repeated also in 998330ac2e7 (read-cache: look for shared index files next to the index, too, 2021-08-26), which we fix here by _not_ mistaking an empty base index for a missing `sharedindex.*` file. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>