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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).

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