CLEANUP: sample: remove uneeded check in json validation

- check functions are never called with a NULL args list, it is always
  an array, so first check can be removed
- the expression parser guarantees that we can't have anything else,
  because we mentioned json converter takes a mandatory string argument.
  Thus test on `ARGT_STR` can be removed as well
- also add breaking line between enum and function declaration

In order to validate it, add a simple json test testing very simple
cases but can be improved in the future:

- default json converter without args
- json converter failing on error (utf8)
- json converter with error being removed (utf8s)

Signed-off-by: William Dauchy <wdauchy@gmail.com>

reg-tests/converter/json.vtc

@@ -0,0 +1,41 @@
+varnishtest "json converter test"
+
+#REQUIRE_VERSION=1.6
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp
+} -repeat 2 -start
+
+haproxy h1 -conf {
+    defaults
+	mode http
+	timeout connect 1s
+	timeout client  1s
+	timeout server  1s
+
+    frontend fe
+	bind "fd@${fe}"
+
+	http-response set-header json0 "%[str(foo 1/2),json]"
+	# bad UTF-8 sequence
+	http-response set-header json1 "%[str(\xE0),json(utf8)]"
+	# bad UTF-8 sequence, but removes them
+	http-response set-header json2 "%[str(-\xE0-),json(utf8s)]"
+
+	default_backend be
+
+    backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq -url "/"
+	rxresp
+	expect resp.http.json0 == "foo 1\\/2"
+	expect resp.http.json1 == ""
+	expect resp.http.json2 == "--"
+	expect resp.status == 200
+} -run

src/sample.c

@@ -2152,21 +2152,12 @@ enum input_type {
 	IT_UTF8P,
 	IT_UTF8PS,
 };
+
 static int sample_conv_json_check(struct arg *arg, struct sample_conv *conv,
                                   const char *file, int line, char **err)
 {
 	enum input_type type;
 
-	if (!arg) {
-		memprintf(err, "Unexpected empty arg list");
-		return 0;
-	}
-
-	if (arg->type != ARGT_STR) {
-		memprintf(err, "Unexpected arg type");
-		return 0;
-	}
-
 	if (strcmp(arg->data.str.area, "") == 0)
 		type = IT_ASCII;
 	else if (strcmp(arg->data.str.area, "ascii") == 0)