fix: allow coderd to start with an empty DERP map when built-in DERP is disabled (#24544)

Allow coderd to start with an empty base DERP map when built-in DERP
is disabled and no static DERP map is configured, so DERP can come from
workspace proxies after startup.

Also add a DERP healthcheck warning when no DERP servers are currently
available at runtime.

Related to: https://linear.app/codercom/issue/PLAT-43/bug-coderd-unable-to-be-started-if-built-in-derp-server-disabled-and
Related to: https://github.com/coder/coder/issues/22324

cli/server_test.go

@@ -2370,27 +2370,26 @@ func TestConnectToPostgres(t *testing.T) {
 	})
 }
 
-func TestServer_InvalidDERP(t *testing.T) {
+func TestServer_DisabledDERP_EmptyBaseMap(t *testing.T) {
 	t.Parallel()
 
+	ctx, cancelFunc := context.WithTimeout(context.Background(), testutil.WaitShort)
+	defer cancelFunc()
+
 	// Try to start a server with the built-in DERP server disabled and no
 	// external DERP map.
-
-	inv, _ := clitest.New(t,
+	inv, cfg := clitest.New(t,
 		"server",
 		dbArg(t),
 		"--http-address", ":0",
 		"--access-url", "http://example.com",
 		"--derp-server-enable=false",
-		"--derp-server-stun-addresses", "disable",
-		"--block-direct-connections",
 	)
-	err := inv.Run()
-	require.Error(t, err)
-	require.ErrorContains(t, err, "A valid DERP map is required for networking to work")
+	clitest.Start(t, inv.WithContext(ctx))
+	waitAccessURL(t, cfg)
 }
 
-func TestServer_DisabledDERP(t *testing.T) {
+func TestServer_DisabledDERP_ExternalMap(t *testing.T) {
 	t.Parallel()
 
 	derpMap, _ := tailnettest.RunDERPAndSTUN(t)