Fixes #88178

Motivation for this change

From the discussion at #88178 :
The nixpkgs version of ofono is patched to read a OFONO_PLUGIN_PATH environment variable. It crashes when this variable is not set.
This also means that the services.ofono service does not work if services.ofono.plugins is empty.

The issue is that the plugin path check is inverted.

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 nixpkgs-review --run "nixpkgs-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.