Do not add null or whitespace lines to payload in ElasticsearchPayloa…

[andrey-kozlov-skuvault]

What issue does this PR address?
This is done to avoid NullReferenceException in Elasticsearch.Net.PostData.Write method in line 204 when we getting bytes from possibly null object var bytes = enumerator.Current.Utf8Bytes();

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit follows our guidelines
• Unit Tests for the changes have been added (for bug fixes / features)

Other information:
These tests fail before that change:

using System;
using System.IO;
using System.Text;
using Elasticsearch.Net;
using FluentAssertions;
using Serilog.Sinks.Elasticsearch.Durable;
using Xunit;

namespace Serilog.Sinks.Elasticsearch.Tests;

public class ElasticsearchPayloadReaderTests : IDisposable
{
    private readonly string _tempFileFullPath;

    public ElasticsearchPayloadReaderTests()
    {
        _tempFileFullPath = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid():N}-{20000101}.json");
    }

    public void Dispose()
    {
        System.IO.File.Delete(_tempFileFullPath);
    }

    [Theory]
    [InlineData("")]
    [InlineData(null)]
    [InlineData(" ")]
    public void ReadPayload_SkipsEmptyLines(string emptyLine)
    {
        // Arrange
        var payloadReader = new ElasticsearchPayloadReader("testPipelineName", "TestTypeName", null,
            (_, _) => "TestIndex", ElasticOpType.Index);
        var lines = new[]
        {
            "line1",
            emptyLine,
            "line2"
        };
        // Important to use UTF8 with BOM if we are starting from 0 position 
        System.IO.File.WriteAllLines(_tempFileFullPath, lines, new UTF8Encoding(true));

        // Act
        var fileSetPosition = new FileSetPosition(0, _tempFileFullPath);
        var count = 0;
        var payload = payloadReader.ReadPayload(int.MaxValue, null, ref fileSetPosition, ref count, _tempFileFullPath);

        // Assert
        // Before the fix, these lines would provide NullReference exception
        var data = PostData.MultiJson(payload);
        data.Write(new MemoryStream(), new ConnectionConfiguration());
    }
}