Do not include embedded content if its not specified. Leave null

src/X.Bluesky/BlueskyClient.cs

@@ -1,4 +1,5 @@
 using System.Collections.Frozen;
+using System.Globalization;
 using System.Net.Http.Headers;
 using System.Security.Authentication;
 using System.Text;
@@ -127,7 +128,7 @@ private async Task CreatePost(string text, Uri? url)
         }
 
         // Fetch the current time in ISO 8601 format, with "Z" to denote UTC
-        var now = DateTime.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ");
+        var now = DateTime.UtcNow.ToString("o", CultureInfo.InvariantCulture);
         var facetBuilder = new FacetBuilder();
 
         var facets = facetBuilder.GetFacets(text);
@@ -212,4 +213,4 @@ private async Task CreatePost(string text, Uri? url)
         // This throws an exception if the HTTP response status is an error code.
         response.EnsureSuccessStatusCode();
     }
-}
+}

src/X.Bluesky/Models/Post.cs

@@ -11,7 +11,7 @@ public record Post
 
     public string CreatedAt { get; set; } = "";
 
-    public Embed Embed { get; set; } = new();
+    public Embed? Embed { get; set; } = null;
 
     public List<string> Langs { get; set; } = new();