Add default for package add

Author: Forgind

src/Cli/dotnet/Commands/Hidden/Add/Package/AddPackageCommandParser.cs

@@ -41,7 +41,7 @@ private static Command ConstructCommand()
             }
             else
             {
-                return new PackageAddCommand(parseResult, parseResult.GetValue(AddCommandParser.ProjectArgument)).Execute();
+                return new PackageAddCommand(parseResult, parseResult.GetValue(AddCommandParser.ProjectArgument) ?? Directory.GetCurrentDirectory()).Execute();
             }
         });