buildDotnetModule: fix running fetch-deps with no nugetDeps defined. This eases the initial setup when creating a package

pkgs/build-support/dotnet/build-dotnet-module/default.nix

···
 
   passthru = {
     inherit nuget-source;
-  } // lib.optionalAttrs (nugetDepsFile != null) {
+  } // lib.optionalAttrs (!lib.isDerivation nugetDeps) {
     fetch-deps =
       let
         flags = dotnetFlags ++ dotnetRestoreFlags;