mariadb-backup with --prepare option could result in empty redo log
file. When --prepare is followed by --prepare --export, we exit early
in srv_start function without opening the ibdata1 tablespace. Later
while trying to read rollback segment header page, we hit the debug
assert which claims that the system space should already have been
opened.

There are two assert cases here.

Issue-1: System tablespace object is not there in fil space hash i.e.
srv_sys_space.open_or_create() is not called.

Issue-2: The system tablespace data file ibdata1 is not opened i.e.
fil_system.sys_space->open() is not called.

Fix: For empty redo log and restore operation, open system tablespace
before returning.