Don't use `GetNonWhiteSpaceString()` with nullable JSON fields

Fixes #747

DiscordChatExporter.Core/Discord/Data/Channel.cs

@@ -45,7 +45,7 @@ public partial record Channel
     public static Channel Parse(JsonElement json, ChannelCategory? category = null, int? position = null)
     {
         var id = json.GetProperty("id").GetNonWhiteSpaceString().Pipe(Snowflake.Parse);
-        var guildId = json.GetPropertyOrNull("guild_id")?.GetNonWhiteSpaceString().Pipe(Snowflake.Parse);
+        var guildId = json.GetPropertyOrNull("guild_id")?.GetStringOrNull()?.Pipe(Snowflake.Parse);
         var topic = json.GetPropertyOrNull("topic")?.GetStringOrNull();
         var kind = (ChannelKind)json.GetProperty("type").GetInt32();
 

DiscordChatExporter.Core/Discord/Data/Emoji.cs

@@ -48,7 +48,7 @@ public partial record Emoji
 
     public static Emoji Parse(JsonElement json)
     {
-        var id = json.GetPropertyOrNull("id")?.GetNonWhiteSpaceString();
+        var id = json.GetPropertyOrNull("id")?.GetStringOrNull();
         var name = json.GetProperty("name").GetNonWhiteSpaceString();
         var isAnimated = json.GetPropertyOrNull("animated")?.GetBoolean() ?? false;