improve: SettingsData.FromJson null safety + GatewayUrlHelper static array (#148)

Two small coding improvements:

1. SettingsData.FromJson(string? json) — null/empty guard
   - Change parameter from non-nullable 'string' to 'string?'
   - Return null immediately for null or empty input rather than
     forwarding to JsonSerializer.Deserialize which would throw
     ArgumentNullException (not caught by the existing JsonException
     handler)
   - Adds two regression tests: null and empty string both return null

2. GatewayUrlHelper.RemoveUserInfo — static readonly char array
   - Replace the inline 'new[] { '/', '?', '#' }' array literal
     in RemoveUserInfo with a 'private static readonly' field
   - The method is called on every URL normalisation and display
     sanitisation; the previous form allocated a new char array on
     each call, which is now eliminated

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/OpenClaw.Shared/GatewayUrlHelper.cs

@@ -6,6 +6,8 @@ public static class GatewayUrlHelper
 {
     public const string ValidationMessage = "Gateway URL must be a valid URL (ws://, wss://, http://, or https://).";
 
+    private static readonly char[] s_authorityTerminators = { '/', '?', '#' };
+
     public static bool IsValidGatewayUrl(string? gatewayUrl) =>
         TryNormalizeWebSocketUrl(gatewayUrl, out _);
 
@@ -141,7 +143,7 @@ private static string RemoveUserInfo(string url)
         }
 
         var authorityStart = schemeSeparator + 3;
-        var authorityEnd = url.IndexOfAny(new[] { '/', '?', '#' }, authorityStart);
+        var authorityEnd = url.IndexOfAny(s_authorityTerminators, authorityStart);
         if (authorityEnd < 0)
         {
             authorityEnd = url.Length;

src/OpenClaw.Shared/SettingsData.cs

@@ -37,8 +37,10 @@ public class SettingsData
 
     public string ToJson() => JsonSerializer.Serialize(this, s_options);
 
-    public static SettingsData? FromJson(string json)
+    public static SettingsData? FromJson(string? json)
     {
+        if (string.IsNullOrEmpty(json))
+            return null;
         try
         {
             return JsonSerializer.Deserialize<SettingsData>(json);

tests/OpenClaw.Tray.Tests/SettingsRoundTripTests.cs

@@ -170,6 +170,14 @@ public void InvalidJson_ReturnsNull()
         Assert.Null(SettingsData.FromJson("not json at all"));
     }
 
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    public void NullOrEmptyJson_ReturnsNull(string? json)
+    {
+        Assert.Null(SettingsData.FromJson(json));
+    }
+
     [Fact]
     public void EmptyUserRules_RoundTrips()
     {