fix write_subcaption_file error when using opengl renderer (#3546)

* fix write_subcaption_file error when using opengl renderer

* Update manim/scene/scene_file_writer.py

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Co-authored-by: Benjamin Hackl <[email protected]>

Author: yuan-xy

manim/scene/scene_file_writer.py

@@ -725,6 +725,8 @@ def flush_cache_directory(self):
 
     def write_subcaption_file(self):
         """Writes the subcaption file."""
+        if config.output_file is None:
+            return
         subcaption_file = Path(config.output_file).with_suffix(".srt")
         subcaption_file.write_text(srt.compose(self.subcaptions), encoding="utf-8")
         logger.info(f"Subcaption file has been written as {subcaption_file}")