[7.0] 1996366df vtc: Test case for #3706

[Nils Goroll]

commit 1996366df42bf5c89e7f2e74a8871f247e4693a3
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Tue Sep 28 17:45:22 2021 +0200

    vtc: Test case for #3706

diff --git a/bin/varnishtest/tests/r03706.vtc b/bin/varnishtest/tests/r03706.vtc
new file mode 100644
index 000000000..582c6f922
--- /dev/null
+++ b/bin/varnishtest/tests/r03706.vtc
@@ -0,0 +1,18 @@
+varnishtest "Null regex subject in ban expression"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend { } -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run
+
+varnish v1 -cliok "ban req.http.nonexistent ~ foo"
+
+client c1 -run