nuget-to-nix: set nullglob

This stops nuget-to-nix from failing when there are no packages in the
output.

pkgs/build-support/dotnet/nuget-to-nix/nuget-to-nix.sh

···
 #!@runtimeShell@
 
 set -euo pipefail
+shopt -s nullglob
 
 export PATH="@binPath@"
 # used for glob ordering of package names