MDEV-31251 MDEV-30968 breaks running mariabackup on older mariadb (opendir(NULL))

The problem happened when running mariabackup agains a pre-MDEV-30971 server,
i.e. not having yet the system variable @@aria_log_dir_path.

As a result, backup_start() called the function backup_files_from_datadir()
with a NULL value, which further caused a crash.

Fix:
Perform this call:

    backup_files_from_datadir(.., aria_log_dir_path, ..)

only if aria_log_dir_path is not NULL. Otherwise,
assume that Aria log files are in their default location,
so they've just copied by the previous call:

    backup_files_from_datadir(.., fil_path_to_mysql_datadir, ..)

Thanks to Walter Doekes for a patch proposal.

extra/mariabackup/backup_copy.cc

@@ -1439,9 +1439,10 @@ bool backup_start(ds_ctxt *ds_data, ds_ctxt *ds_meta,
 
         if (!backup_files_from_datadir(ds_data, fil_path_to_mysql_datadir,
 	                               "aws-kms-key") ||
-            !backup_files_from_datadir(ds_data,
-                                       aria_log_dir_path,
-                                       "aria_log")) {
+            (aria_log_dir_path &&
+             !backup_files_from_datadir(ds_data,
+                                        aria_log_dir_path,
+                                        "aria_log"))) {
 		return false;
 	}