java classpath hook: allow unbound $CLASSPATH

[eraserhd]

Motivation for this change

Started getting CLASSPATH: unbound from this hook after updating master a couple hours ago, and it seems to be related to #72347.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @Ericson2314

This seems to cause rebuilds of JVMs, unforunately; but we can't build Java/Graal packages anymore without it.

[Ericson2314]

Thanks!