internal/shaderlister: output [] instead of null when no shaders are found

Updates #3157

internal/shaderlister/main.go

@@ -56,13 +56,15 @@ func xmain() error {
 	}
 
 	pkgs, err := packages.Load(&packages.Config{
-		Mode: packages.NeedName | packages.NeedTypes | packages.NeedImports | packages.NeedDeps | packages.NeedSyntax | packages.NeedTypesInfo,
+		Mode: packages.NeedName | packages.NeedImports | packages.NeedDeps | packages.NeedTypes | packages.NeedSyntax | packages.NeedTypesInfo,
 	}, flag.Args()...)
 	if err != nil {
 		return err
 	}
 
-	var shaders []Shader
+	// Collect shader information.
+	// Even if no shader is found, the output should be a JSON array. Start with an empty slice, not nil.
+	shaders := []Shader{}
 
 	packages.Visit(pkgs, func(pkg *packages.Package) bool {
 		path := pkg.PkgPath

internal/shaderlister/shaderlister_test.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"os/exec"
 	"slices"
+	"strings"
 	"testing"
 )
 
@@ -63,3 +64,19 @@ func TestRun(t *testing.T) {
 		}
 	}
 }
+
+func TestEmpty(t *testing.T) {
+	cmd := exec.Command("go", "run", "github.com/hajimehoshi/ebiten/v2/internal/shaderlister", "github.com/ebitengine/purego")
+	out, err := cmd.Output()
+	if err != nil {
+		if err, ok := err.(*exec.ExitError); ok {
+			t.Fatalf("Error: %v\n%s", err, err.Stderr)
+		}
+		t.Fatal(err)
+	}
+
+	// Check the output is `[]`, not `null`.
+	if got, want := strings.TrimSpace(string(out)), "[]"; got != want {
+		t.Errorf("output: got: %q, want: %q", got, want)
+	}
+}