fix(tsconnect): avoid nil services slice in netmap JSON

userServicesFromView returned a nil slice when a node advertised no
services, which (combined with the omitempty tag) caused the
services field to be dropped or serialize as null instead of [].
TypeScript declares services as a non-optional array, so JS callers
calling .find()/.some() on it would throw intermittently depending on
which netmap snapshot they observed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -1393,7 +1393,7 @@ func (i *jsIPN) setServices(jsServices js.Value) js.Value {
 // userServicesFromView converts a hostinfo services slice to jsService entries,
 // filtering out internal peerapi protocol entries (already reflected in peerAPIURL).
 func userServicesFromView(svcs views.Slice[tailcfg.Service]) []jsService {
-	var out []jsService
+	out := make([]jsService, 0, svcs.Len())
 	for _, s := range svcs.All() {
 		switch s.Proto {
 		case tailcfg.PeerAPI4, tailcfg.PeerAPI6, tailcfg.PeerAPIDNS:
@@ -1652,7 +1652,7 @@ type jsNetMapNode struct {
 	MachineKey string      `json:"machineKey"`
 	NodeKey    string      `json:"nodeKey"`
 	PeerAPIURL string      `json:"peerAPIURL,omitempty"`
-	Services   []jsService `json:"services,omitempty"`
+	Services   []jsService `json:"services"`
 }
 
 type jsNetMapSelfNode struct {