tests/modules/kafka: exercise the null keyword on the xlat key arg

Now that tmpl parsing recognises `null` as an explicit FR_TYPE_NULL
placeholder, swap the zero-length-value produce's key from `''` to
`null` so the test doubles as regression coverage for the keyword
end-to-end (tokenize -> xlat arg list -> cast to octets -> zero-
length key -> "no key on the wire").

Author: arr2036

src/tests/modules/kafka/xlat.unlang

@@ -44,7 +44,7 @@ if (!%kafka.produce('freeradius-test-xlat-alt', "xlat-key", "alternate topic"))
 #
 #  Empty value - librdkafka accepts zero-length payloads.
 #
-if (!%kafka.produce('freeradius-test-xlat', '', "")) {
+if (!%kafka.produce('freeradius-test-xlat', null, "")) {
 	test_fail
 }