feat: add Empty method and fix link handling

Added a static `Empty` method to `DownloadEnumerable` for
creating an empty instance. Updated link handling in
`SequentialDownloader` to use `AbsoluteUri` instead of
`ToString()`, ensuring correct link representation.

These changes improve usability and ensure consistency in
link formatting.

aeqw89.Beam/aeqw89.Beam.csproj

@@ -7,7 +7,7 @@
         <Title>Beam</Title>
         <Authors>aeqw89</Authors>
         <Company>qwsdcvghyu</Company>
-        <Version>1.3.2</Version>
+        <Version>1.3.6</Version>
         <Description>A library for downloading internet resources</Description>
         <PackageProjectUrl>https://github.com/qwsdcvghyu89/Beam</PackageProjectUrl>
         <RepositoryUrl>https://github.com/qwsdcvghyu89/Beam</RepositoryUrl>