Fix bash syntax error in backup_folder return logic

The arithmetic test `(( rc == 0 || (rc == 1 && -z "$filtered_output") ))`
breaks the bash parser when filtered_output is a multiline string. Split
the test so the -z check runs in a [[ ]] context instead.

k8s/automated-nfs-backup.sh

@@ -415,7 +415,9 @@ backup_folder() {
     if [[ -n "$filtered_output" ]]; then
         log_warn "7z output for '${folder_path}': ${filtered_output}"
     fi
-    if (( rc == 0 || (rc == 1 && -z "$filtered_output") )); then
+    if (( rc == 0 )); then
+        return 0
+    elif (( rc == 1 )) && [[ -z "$filtered_output" ]]; then
         return 0
     fi
     return 1