Make EnvironmentFile conditional in systemd service

Only include EnvironmentFile when at least one *File option
(openaiApiKeyFile, tokenFile, qdrantApiKeyFile) is set.
Without this, an empty EnvironmentFile list causes systemd
to fail with 'Failed to load environment files'.

flake.nix

@@ -129,6 +129,8 @@
                 WorkingDirectory = "/var/lib/st-pierre";
                 StateDirectory = "st-pierre";
                 ExecStart = "${st-pierre-pkg}/bin/st-pierre";
+              }
+              // lib.mkIf (cfg.openaiApiKeyFile != null || cfg.tokenFile != null || cfg.qdrantApiKeyFile != null) {
                 EnvironmentFile = lib.optional (cfg.openaiApiKeyFile != null) cfg.openaiApiKeyFile
                   ++ lib.optional (cfg.tokenFile != null) cfg.tokenFile
                   ++ lib.optional (cfg.qdrantApiKeyFile != null) cfg.qdrantApiKeyFile;