journal: do not trigger assertion when journal_file_close() get NULL

We generally expect destructors to not complain if a NULL argument is passed.

Closes #12400.

src/journal/journal-file.c

@@ -340,7 +340,8 @@ bool journal_file_is_offlining(JournalFile *f) {
 }
 
 JournalFile* journal_file_close(JournalFile *f) {
-        assert(f);
+        if (!f)
+                return NULL;
 
 #if HAVE_GCRYPT
         /* Write the final tag */

src/journal/journald-server.c

@@ -2232,11 +2232,8 @@ void server_done(Server *s) {
 
         client_context_flush_all(s);
 
-        if (s->system_journal)
-                (void) journal_file_close(s->system_journal);
-
-        if (s->runtime_journal)
-                (void) journal_file_close(s->runtime_journal);
+        (void) journal_file_close(s->system_journal);
+        (void) journal_file_close(s->runtime_journal);
 
         ordered_hashmap_free_with_destructor(s->user_journals, journal_file_close);