Fix warnings

Author: inyutin-maxim

System.IO.Abstractions.Analyzers/CodeActions/FileServiceConstructorInitialCodeAction.cs

@@ -55,14 +55,14 @@ protected override async Task<Document> GetChangedDocumentAsync(CancellationToke
 
 		var fileSystem = RoslynClassFileSystem.GetUsing(compilationUnitSyntax, Constants.FileSystemNameSpace);
 
-		if (fileSystem != default(UsingDirectiveSyntax))
+		if (fileSystem != null)
 		{
 			return editor.GetChangedDocument();
 		}
 
 		var systemIo = RoslynClassFileSystem.GetSystemIoUsing(compilationUnitSyntax);
 
-		if (systemIo == default(UsingDirectiveSyntax))
+		if (systemIo == null)
 		{
 			editor.InsertBefore(compilationUnitSyntax.Usings.FirstOrDefault(),
 				RoslynClassFileSystem.GetFileSystemUsing());

System.IO.Abstractions.Analyzers/CodeActions/FileServiceInterfaceInjectionCodeAction.cs

@@ -56,14 +56,14 @@ protected override async Task<Document> GetChangedDocumentAsync(CancellationToke
 
 		var fileSystem = RoslynClassFileSystem.GetUsing(compilationUnitSyntax, Constants.FileSystemNameSpace);
 
-		if (fileSystem != default(UsingDirectiveSyntax))
+		if (fileSystem != null)
 		{
 			return editor.GetChangedDocument();
 		}
 
 		var systemIo = RoslynClassFileSystem.GetSystemIoUsing(compilationUnitSyntax);
 
-		if (systemIo == default(UsingDirectiveSyntax))
+		if (systemIo == null)
 		{
 			editor.InsertBefore(compilationUnitSyntax.Usings.FirstOrDefault(),
 				RoslynClassFileSystem.GetFileSystemUsing());

System.IO.Abstractions.Analyzers/CodeFixes/BaseInvokeCodeFix.cs

@@ -23,6 +23,11 @@ public override async sealed Task RegisterCodeFixesAsync(CodeFixContext context)
 		var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken)
 			.ConfigureAwait(false);
 
+		if (root == null)
+		{
+			return;
+		}
+
 		var classDeclaration = root.FindNode(context.Span)
 			.FirstAncestorOrSelf<ClassDeclarationSyntax>();