Fix issues preventing downloads

[UrbanCMC]

Fixes the following issues:

• Building an HttpRequestMessage when no cookies are available causes a download error

00:02:55.089 [ERROR] (Wabbajack.Installer.StandardInstaller) Download error for file Draugr Upgrades and Improvements (latest)-54930-2-4-2-1639244041.7z|System.FormatException: The format of value '' is invalid.
   at System.Net.Http.Headers.HttpHeaderParser.ParseValue(String value, Object storeValue, Int32& index)
   at System.Net.Http.Headers.HttpHeaders.ParseAndAddValue(HeaderDescriptor descriptor, HeaderStoreItemInfo info, String value)
   at System.Net.Http.Headers.HttpHeaders.Add(HeaderDescriptor descriptor, String value)
   at Wabbajack.Common.HttpExtensions.AddCookies(HttpRequestMessage msg, Cookie[] cookies) in C:\oss\wabbajack\Wabbajack.Common\HttpExtensions.cs:line 21
   at Wabbajack.Common.HttpExtensions.ToHttpRequestMessage(BrowserDownloadState browserState) in C:\oss\wabbajack\Wabbajack.Common\HttpExtensions.cs:line 44
   at Wabbajack.Downloaders.NexusDownloader.DownloadManually(Archive archive, Nexus state, AbsolutePath destination, IJob job, CancellationToken token) in C:\oss\wabbajack\Wabbajack.Downloaders.Nexus\NexusDownloader.cs:line 191
   at Wabbajack.Downloaders.NexusDownloader.Download(Archive archive, Nexus state, AbsolutePath destination, IJob job, CancellationToken token) in C:\oss\wabbajack\Wabbajack.Downloaders.Nexus\NexusDownloader.cs:line 123
   at Wabbajack.Downloaders.DownloadDispatcher.Download(Archive a, AbsolutePath dest, Job`1 job, CancellationToken token, Nullable`1 useProxy) in C:\oss\wabbajack\Wabbajack.Downloaders.Dispatcher\DownloadDispatcher.cs:line 112
   at Wabbajack.Downloaders.DownloadDispatcher.Download(Archive a, AbsolutePath dest, CancellationToken token, Nullable`1 proxy) in C:\oss\wabbajack\Wabbajack.Downloaders.Dispatcher\DownloadDispatcher.cs:line 54
   at Wabbajack.Downloaders.DownloadDispatcher.DownloadWithPossibleUpgrade(Archive archive, AbsolutePath destination, CancellationToken token) in C:\oss\wabbajack\Wabbajack.Downloaders.Dispatcher\DownloadDispatcher.cs:line 155
   at Wabbajack.Installer.AInstaller`1.DownloadArchive(Archive archive, Boolean download, CancellationToken token, Nullable`1 destination) in C:\oss\wabbajack\Wabbajack.Installer\AInstaller.cs:line 399

• Trying to parse an invalid .download_package throws an exception on every retry, making a download impossible

00:00:22.565 [ERROR] (Wabbajack.Networking.Http.SingleThreadedDownloader) Failed to download 'Unofficial Skyrim Creation Club Content Patch-18975-7-7-1691284489.zip' after 3 tries.|System.Text.Json.JsonException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
 ---> System.Text.Json.JsonReaderException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.
   at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
   at System.Text.Json.Utf8JsonReader.Read()
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   --- End of inner exception stack trace ---
   at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& state, JsonReaderException ex)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo`1 jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
   at Wabbajack.Networking.Http.ResumableDownloader.LoadPackage() in C:\oss\wabbajack\Wabbajack.Networking.Http\ResumableDownloader.cs:line 169
   at Wabbajack.Networking.Http.ResumableDownloader.Download(CancellationToken token) in C:\oss\wabbajack\Wabbajack.Networking.Http\ResumableDownloader.cs:line 50
   at Wabbajack.Networking.Http.SingleThreadedDownloader.Download(HttpRequestMessage message, AbsolutePath outputPath, IJob job, CancellationToken token) in C:\oss\wabbajack\Wabbajack.Networking.Http\SingleThreadedDownloader.cs:line 41