BUG/MINOR: sample: Make the `field` converter compatible with `-m found`

Previously an expression like:

    path,field(2,/) -m found

always returned `true`.

Bug exists since the `field` converter exists. That is:
f399b0debfc6c7dc17c6ad503885c911493add56

The fix should be backported to 1.6+.

(cherry picked from commit 4381d26edc03faa46401eb0fe82fd7be84be14fd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/reg-tests/converter/field.vtc b/reg-tests/converter/field.vtc
new file mode 100644
index 0000000..29608fe
--- /dev/null
+++ b/reg-tests/converter/field.vtc
@@ -0,0 +1,39 @@
+varnishtest "field converter Test"
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp
+} -repeat 3 -start
+
+haproxy h1 -conf {
+    defaults
+	mode http
+	timeout connect 1s
+	timeout client  1s
+	timeout server  1s
+
+    frontend fe
+	bind "fd@${fe}"
+
+	#### requests
+	http-request set-var(txn.uri) path
+	http-response set-header Found %[var(txn.uri),field(3,/)] if { var(txn.uri),field(3,/) -m found }
+
+	default_backend be
+
+    backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq -url "/foo/bar/baz"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "bar"
+	txreq -url "/foo"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "<undef>"
+} -run