fix(librarian): prevent panic on release command without subcommand (#1901)

Fixes #1723

When running  without a subcommand, the program
panicked due to a nil pointer dereference. This occurred because the
command has no action and no configuration.

This change adds a check to see if a command has a function. If it does
not, it prints the command's help text and exits, preventing the panic.

With the fix:
```
jinseop@jinseop:/workspace/librarian$ go run ./cmd/librarian release
Manages releases of libraries.

Usage:
  librarian release <command> [arguments]

Commands:

  init                       initiates a release by creating a release pull request.
  tag-and-release            tags and creates a GitHub release for a merged pull request.

2025/09/02 16:55:30 command "release" requires a subcommand
exit status 1
```

Author: jinseopkim0

internal/librarian/librarian.go

@@ -59,6 +59,14 @@ func Run(ctx context.Context, arg ...string) error {
 	if err != nil {
 		return err
 	}
+
+	// If a command is just a container for subcommands, it won't have a
+	// Run function. In that case, display its usage instructions.
+	if cmd.Run == nil {
+		cmd.Flags.Usage()
+		return fmt.Errorf("command %q requires a subcommand", cmd.Name())
+	}
+
 	if err := cmd.Parse(arg); err != nil {
 		// We expect that if cmd.Parse fails, it will already
 		// have printed out a command-specific usage error,

internal/librarian/librarian_test.go

@@ -15,12 +15,14 @@
 package librarian
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"math/rand"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/go-git/go-git/v5"
@@ -42,6 +44,44 @@ func TestRun(t *testing.T) {
 	}
 }
 
+func TestParentCommands(t *testing.T) {
+	ctx := context.Background()
+
+	tests := []struct {
+		name       string
+		command    string
+		wantErr    bool
+		wantErrMsg string // Expected substring in the error
+	}{
+		{
+			name:       "release no subcommand",
+			command:    "release",
+			wantErr:    true,
+			wantErrMsg: `command "release" requires a subcommand`,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			err := Run(ctx, test.command)
+
+			if test.wantErr {
+				if err == nil {
+					t.Fatalf("Run(ctx, %q) got nil, want error containing %q", test.command, test.wantErrMsg)
+				}
+				if !strings.Contains(err.Error(), test.wantErrMsg) {
+					t.Errorf("Run(ctx, %q) got error %q, want error containing %q", test.command, err.Error(), test.wantErrMsg)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Run(ctx, %q) got error %v, want nil", test.command, err)
+			}
+		})
+	}
+}
+
 func TestIsURL(t *testing.T) {
 	for _, test := range []struct {
 		name  string