fix: report no timestamp when a task's sources match no file

TimestampChecker.Value returned a sentinel for "no source found": first
the string "0", then time.Unix(0, 0) once the value became a time.Time
exposed to templates. Rendered, that sentinel reads as a genuine date,
1970-01-01 01:00:00 +0100 CET, whose zone depends on the machine — so a
command interpolating {{.TIMESTAMP}} gets a plausible but meaningless
multi-word value instead of nothing.

Return an empty string, like NoneChecker already does for a checker with
nothing to report.

internal/fingerprint/sources_timestamp.go

@@ -122,8 +122,12 @@ func (checker *TimestampChecker) Value(t *ast.Task) (any, error) {
 		return time.Now(), err
 	}
 
+	// No source matched, so there is no modification time to report. Returning
+	// the Unix epoch here would render as a real date in templates, e.g.
+	// "1970-01-01 01:00:00 +0100 CET", which reads as a genuine timestamp and
+	// carries a machine-dependent zone.
 	if sourcesMaxTime.IsZero() {
-		return time.Unix(0, 0), nil
+		return "", nil
 	}
 
 	return sourcesMaxTime, nil

internal/fingerprint/sources_timestamp_test.go

@@ -0,0 +1,44 @@
+package fingerprint
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/go-task/task/v3/taskfile/ast"
+)
+
+func TestTimestampCheckerValue(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "source.txt"), []byte("content"), 0o644))
+	checker := NewTimestampChecker(t.TempDir(), true)
+
+	t.Run("reports the newest source modification time", func(t *testing.T) {
+		t.Parallel()
+
+		value, err := checker.Value(&ast.Task{
+			Dir:     dir,
+			Sources: []*ast.Glob{{Glob: "source.txt"}},
+		})
+		require.NoError(t, err)
+		assert.IsType(t, time.Time{}, value)
+		assert.False(t, value.(time.Time).IsZero())
+	})
+
+	t.Run("reports no value when no source matches", func(t *testing.T) {
+		t.Parallel()
+
+		value, err := checker.Value(&ast.Task{
+			Dir:     dir,
+			Sources: []*ast.Glob{{Glob: "gen/**/*.go"}},
+		})
+		require.NoError(t, err)
+		assert.Equal(t, "", value)
+	})
+}