Fix some nullable warnings in DFS library.

src/NameServer/NameServer.csproj

@@ -4,7 +4,7 @@
     <OutputType>Exe</OutputType>
     <TargetFramework>net6.0</TargetFramework>
     <CodeAnalysisRuleSet>../Jumbo.ruleset</CodeAnalysisRuleSet>
-    <Nullable>enable</Nullable>
+    <Nullable>annotations</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/Ookii.Jumbo.Dfs/DfsInputStream.cs

@@ -53,10 +53,10 @@ public DfsInputStream(INameServerClientProtocol nameServer, string path)
 
             _nameServer = nameServer;
             _log.DebugFormat("Opening file {0} from the DFS.", path);
-            _file = nameServer.GetFileInfo(path);
             // GetFileInfo doesn't throw if the file doesn't exist; we do.
-            if (_file == null)
+            _file = nameServer.GetFileInfo(path) ??
                 throw new FileNotFoundException(string.Format(System.Globalization.CultureInfo.CurrentCulture, "The file '{0}' does not exist on the distributed file system.", path));
+
             BlockSize = (int)_file.BlockSize;
             _endOffset = _file.Size;
         }

src/Ookii.Jumbo.Dfs/FileSystem/DfsClient.cs

@@ -171,7 +171,7 @@ public override void CreateDirectory(string path)
         /// <returns>
         /// A <see cref="JumboDirectory"/> object representing the directory, or <see langword="null"/> if the directory doesn't exist.
         /// </returns>
-        public override JumboDirectory GetDirectoryInfo(string path)
+        public override JumboDirectory? GetDirectoryInfo(string path)
         {
             return _nameServer.GetDirectoryInfo(path);
         }
@@ -183,7 +183,7 @@ public override JumboDirectory GetDirectoryInfo(string path)
         /// <returns>
         /// A <see cref="JumboFile"/> object referring to the file, or <see langword="null"/> if the file doesn't exist.
         /// </returns>
-        public override JumboFile GetFileInfo(string path)
+        public override JumboFile? GetFileInfo(string path)
         {
             return _nameServer.GetFileInfo(path);
         }
@@ -195,7 +195,7 @@ public override JumboFile GetFileInfo(string path)
         /// <returns>
         /// A <see cref="JumboFileSystemEntry"/> object referring to the file or directory, or <see langword="null"/> if the entry doesn't exist.
         /// </returns>
-        public override JumboFileSystemEntry GetFileSystemEntryInfo(string path)
+        public override JumboFileSystemEntry? GetFileSystemEntryInfo(string path)
         {
             return _nameServer.GetFileSystemEntryInfo(path);
         }