keychain: fix edge-cases in nushell integration

- If the parse does not match/returns an empty list, transpose does
  not transform it into a record and thus load-env fails.

- The parsing assumes keychain to emit bash-style completions, so set
  SHELL=bash to ensure keychain output is in the expected format.

PR #4459

modules/programs/keychain.nix

@@ -117,7 +117,10 @@ in {
       eval "$(SHELL=zsh ${shellCommand})"
     '';
     programs.nushell.extraConfig = mkIf cfg.enableNushellIntegration ''
-      ${shellCommand} | parse -r '(\w+)=(.*); export \1' | transpose -ird | load-env
+      let keychain_shell_command = (SHELL=bash ${shellCommand}| parse -r '(\w+)=(.*); export \1' | transpose -ird)
+      if not ($keychain_shell_command|is-empty) {
+        $keychain_shell_command | load-env
+      }
     '';
     xsession.initExtra = mkIf cfg.enableXsessionIntegration ''
       eval "$(SHELL=bash ${shellCommand})"