An empty media_ids takes an edited post's media away

A JSON body's empty array flattened to nothing, so `media_ids: []` on an edit read as "media not sent" and the
post kept every attachment, where Mastodon removes them. An empty array is now a parameter with no values: the edit
leaves the post without media, and they go to the trash. MediaLifecycleTests checks it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Http/MediaLifecycleTests.cs

@@ -101,9 +101,16 @@ namespace PrivaPub.Tests.Http
 			Assert.Equal("it", edited.Body.Text("language"));
 			Assert.Equal("a red square", (await DB.Default.Find<MediaAttachment>().OneAsync(id, Token)).Description);
 
-			// an edit that doesn't send media keeps them, described as they were
+			// an edit that doesn't send media keeps them, described as they were; an empty media_ids takes them away
 			var kept = (await alice.Client.Put($"/api/v1/statuses/{statusId}", ("status", "media kept"))).Ok();
 			Assert.Equal("a red square", kept.Body["media_attachments"]![0]!.Text("description"));
+			var bare = (await alice.Client.Json(HttpMethod.Put, $"/api/v1/statuses/{statusId}", new System.Text.Json.Nodes.JsonObject
+			{
+				["status"] = "no picture now",
+				["media_ids"] = new System.Text.Json.Nodes.JsonArray()
+			})).Ok();
+			Assert.Empty(bare.Body["media_attachments"]!.AsArray());
+			Assert.Equal("edited out", (await DB.Default.Find<MediaAttachment>().OneAsync(id, Token)).TrashReason);
 		}
 
 		[Fact]

PrivaPub/Api/Mastodon/Infrastructure/MastodonParams.cs

@@ -89,6 +89,9 @@ namespace PrivaPub.Api.Mastodon.Infrastructure
 						Flatten(values, prefix == default ? property.Name : $"{prefix}[{property.Name}]", property.Value);
 					break;
 				case JsonValueKind.Array:
+					// an empty array is still said: media_ids: [] on an edit takes every attachment away
+					if (prefix != default && element.GetArrayLength() == 0)
+						values.TryAdd(prefix, new List<string>());
 					foreach (var item in element.EnumerateArray())
 					{
 						if (item.ValueKind is JsonValueKind.Object or JsonValueKind.Array)