android: treat Tailcft.Node.Created field as optional

fixed tailscale/corp#35019

The Created field is optional from the backend.  OSS changes
in 1.91/2 have removed this in some cases (notably, it no longer
appears when using Headscale).  It is unused in the client.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

android/src/main/java/com/tailscale/ipn/ui/model/TailCfg.kt

@@ -79,7 +79,6 @@ class Tailcfg {
       var AllowedIPs: List<Prefix>? = null,
       var Endpoints: List<String>? = null,
       var Hostinfo: Hostinfo,
-      var Created: Time,
       var LastSeen: Time? = null,
       var Online: Boolean? = null,
       var Capabilities: List<String>? = null,