nixos/nginx: Allow empty port for listen directive

When listening on unix sockets, it doesn't make sense to specify a port
for nginx's listen directive.

Since nginx defaults to port 80 when the port isn't specified (but the
address is), we can change the default for the option to null as well
without changing any behaviour.

nixos/tests/nginx-unix-socket.nix

@@ -0,0 +1,27 @@
+import ./make-test-python.nix ({ pkgs, ... }:
+let
+  nginxSocketPath = "/var/run/nginx/test.sock";
+in
+{
+  name = "nginx-unix-socket";
+
+  nodes = {
+    webserver = { pkgs, lib, ... }: {
+      services.nginx = {
+        enable = true;
+        virtualHosts.localhost = {
+          serverName = "localhost";
+          listen = [{ addr = "unix:${nginxSocketPath}"; }];
+          locations."/test".return = "200 'foo'";
+        };
+      };
+    };
+  };
+
+  testScript = ''
+    webserver.wait_for_unit("nginx")
+    webserver.wait_for_open_unix_socket("${nginxSocketPath}")
+
+    webserver.succeed("curl --fail --silent --unix-socket '${nginxSocketPath}' http://localhost/test | grep '^foo$'")
+  '';
+})