testsuite: test that NUL can be sent over pty

Problem: There's no test that ensures NUL can be transmitted over
a pty allocated for a job.

Add a test to t2612-job-shell-pty.t that sends a NUL as input to a
job with a pty and ensures it is received by the job task.

t/t2612-job-shell-pty.t

@@ -128,4 +128,14 @@ test_expect_success 'pty: no hang when invalid command is run under pty' '
 	test_expect_code 127 run_timeout 15 \
 		flux run -o pty.interactive nosuchcommand
 '
+# Note: test below uses printf(1) to send [Ctrl-SPACE (NUL), newline, Ctrl-D]
+# over the pty connection and expects ^@ to appear in output.
+nul_ctrl_d() {
+	python3 -c 'print("\x00\n\x04", end=None)'
+}
+test_expect_success 'pty: NUL (Ctrl-SPACE) character not dropped' '
+	nul_ctrl_d | flux run -vvv -o pty.interactive -o pty.capture cat -v &&
+	flux job eventlog -HLp output $(flux job last) &&
+	flux job eventlog -HLp output $(flux job last) | grep \\^@
+'
 test_done