tools: bpftool: preserve JSON for batch mode when dumping insns to file

Print a "null" JSON object to standard output when bpftool is used to
print program instructions to a file, so as to avoid breaking JSON
output on batch mode.

This null object was added for most commands in a previous commit, but
this specific case had been omitted.

Fixes: 004b45c0e5 ("tools: bpftool: provide JSON output for all possible commands")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/prog.c

@@ -774,6 +774,9 @@ static int do_dump(int argc, char **argv)
 			      n < 0 ? strerror(errno) : "short write");
 			goto err_free;
 		}
+
+		if (json_output)
+			jsonw_null(json_wtr);
 	} else {
 		if (member_len == &info.jited_prog_len) {
 			const char *name = NULL;