Allow InnoDB start up with empty ib_logfile0 from mariabackup --prepare

A prepared backup from Mariabackup 10.2 does not really need to contain a redo log file, because all redo log will have been applied to the data files.

When the user copies a prepared backup to a data directory (overwriting existing files), it could happen that the data directory already contained redo log files from the past. mariabackup --copy-back) would delete the old redo log files, but a user’s own copying script might not do that. To prevent corruption caused by mixing an old redo log file with data files from a backup, in MDEV-13311 Mariabackup would create a zero-length ib_logfile0 that would prevent startup.

Actually, there is no need to prevent InnoDB from starting up when a single zero-length file ib_logfile0 is present. Only if there exist multiple data files of different lengths, then we should refuse to start up. A single zero-length ib_logfile0 should be treated just like a missing log file: create new log files according to the configuration.