Fix serialisation of empty and single item Fields instances (#8018) (#8026)

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

Author: github-actions[bot]

src/Elastic.Clients.Elasticsearch.Shared/Core/Infer/Fields/Fields.cs

@@ -27,7 +27,7 @@ public sealed class Fields : IUrlParameter, IEnumerable<Field>, IEquatable<Field
 
 	internal Fields() => ListOfFields = new List<Field>();
 
-	internal Fields(IEnumerable<Field> fieldNames) => ListOfFields = fieldNames.ToList();
+	internal Fields(IEnumerable<Field> fieldNames) => ListOfFields = new List<Field>(fieldNames);
 
 	private string DebugDisplay =>
 		$"Count: {ListOfFields.Count} [" +

src/Elastic.Clients.Elasticsearch.Shared/Core/Infer/Fields/FieldsConverter.cs

@@ -22,7 +22,8 @@ internal sealed class FieldsConverter : JsonConverter<Fields>
 			Fields fields = reader.GetString();
 			return fields;
 		}
-		else if (reader.TokenType == JsonTokenType.StartArray)
+
+		if (reader.TokenType == JsonTokenType.StartArray)
 		{
 			var fields = new List<Field>();
 			while (reader.Read() && reader.TokenType != JsonTokenType.EndArray)
@@ -45,6 +46,19 @@ public override void Write(Utf8JsonWriter writer, Fields value, JsonSerializerOp
 			return;
 		}
 
+		if (value.ListOfFields.Count == 0)
+		{
+			writer.WriteStartObject();
+			writer.WriteEndObject();
+			return;
+		}
+
+		if (value.ListOfFields.Count == 1)
+		{
+			JsonSerializer.Serialize(writer, value.ListOfFields[0], options);
+			return;
+		}
+
 		writer.WriteStartArray();
 		foreach (var field in value.ListOfFields)
 		{