A NullPointerException is thrown when trying to create or delete
a snapshot in a repository that has been written to by an older
Elasticsearch after writing to it with a newer Elasticsearch version.
This is because the way snapshots are formatted in the repository
snapshots index file changed in #24477.
This commit changes the parsing of the repository index file so that
it now detects a corrupted index file and fails early the snapshot
operation.
closes#29052
63148dd9ba