fix: nil-pointer panic in WriteToSSA when Metadata is nil (#145)

* fix: nil-pointer panic in WriteToSSA when Metadata is nil

WriteToSSA dereferenced s.Metadata unconditionally when computing the
v4plus flag, but Metadata is an optional *Metadata that only ReadFromSSA
populates. Reading any other format (WebVTT, SRT, TTML, STL) and writing
SSA therefore panicked with a nil-pointer dereference.

Guard the read: nil Metadata now yields v4plus=false (legacy [V4 Styles]
output), matching the existing nil-safe newSSAScriptInfo one line above.

Adds TestWriteToSSANilMetadata as a regression test.

* Address review feedback on TestWriteToSSANilMetadata

Move the nil-metadata comment above the subtitles literal and simplify
its wording, and replace the post-write asserts with assert.NotPanics
to focus the test on the regression (writing SSA no longer panics).

ssa.go

@@ -1180,7 +1180,7 @@ func (s Subtitles) WriteToSSA(o io.Writer) (err error) {
 		return
 	}
 
-	var v4plus = s.Metadata.SSAScriptType == "v4.00+"
+	var v4plus = s.Metadata != nil && s.Metadata.SSAScriptType == "v4.00+"
 
 	// Write Styles block
 	if len(s.Styles) > 0 {

ssa_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io/ioutil"
 	"testing"
+	"time"
 
 	"github.com/asticode/go-astikit"
 	"github.com/asticode/go-astisub"
@@ -122,6 +123,24 @@ func TestSSAv4plus(t *testing.T) {
 	assert.Equal(t, string(c), w.String())
 }
 
+func TestWriteToSSANilMetadata(t *testing.T) {
+	// Metadata intentionally left nil
+	s := &astisub.Subtitles{
+		Items: []*astisub.Item{
+			{
+				StartAt: time.Second,
+				EndAt:   3 * time.Second,
+				Lines:   []astisub.Line{{Items: []astisub.LineItem{{Text: "Hello world"}}}},
+			},
+		},
+	}
+
+	w := &bytes.Buffer{}
+	assert.NotPanics(t, func() {
+		s.WriteToSSA(w)
+	})
+}
+
 func TestInBetweenSSAEffect(t *testing.T) {
 	s, err := astisub.ReadFromSSA(bytes.NewReader([]byte(`[Events]
 Format: Marked, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text