Some test_json_patch test cases have neither a comment field and
nor a error field, so the expression
"comment ? comment : error_s" evaluates to NULL.
The printf statement
printf("Testing '%s', ...", comment ? comment : error_s, ...);
passes NULL as a %s argument. GNU libc handles this gracefully and prints (null).
On AIX, passing NULL for %s prints an empty string, causing the test output
to differ from the expected file.
The fix adds a simple macro:
define STR_OR_NULL(s) ((s) ? (s) : "(null)")
that retruns "(null)" when s is NULL.
