fix: Handle nil value in CleanupNetwork (#2928)

The godoc of `CleanupNetwork` states that a `nil` network will result in
a no-op.

network/network_test.go

@@ -440,3 +440,8 @@ func TestWithNewNetworkContextTimeout(t *testing.T) {
 	require.Empty(t, req.Networks)
 	require.Empty(t, req.NetworkAliases)
 }
+
+func TestCleanupWithNil(t *testing.T) {
+	var network *testcontainers.DockerNetwork
+	testcontainers.CleanupNetwork(t, network)
+}

testing.go

@@ -83,7 +83,9 @@ func CleanupNetwork(tb testing.TB, network Network) {
 	tb.Helper()
 
 	tb.Cleanup(func() {
-		noErrorOrIgnored(tb, network.Remove(context.Background()))
+		if !isNil(network) {
+			noErrorOrIgnored(tb, network.Remove(context.Background()))
+		}
 	})
 }