journal: add missing initialization

Otherwise, SplitMode= in journald.conf is always ignored.

Fixes a regression caused by f48cf2a96d (v258).
Fixes #39046.

src/journal/journald-config.c

@@ -59,6 +59,7 @@ void journal_config_set_defaults(JournalConfig *c) {
                 .max_level_console = -1,
                 .max_level_wall = -1,
                 .max_level_socket = -1,
+                .split_mode = _SPLIT_INVALID,
         };
 
         journal_reset_metrics(&c->system_storage_metrics);