Regression: #6547 main needs default (#6571)

Fix regression error from #6547.

Default value `None` is needed for main function, was removed
accidentally.

Author: snejus

beets/ui/__init__.py

@@ -902,7 +902,7 @@ def _open_library(config: confuse.LazyConfig) -> library.Library:
     return lib
 
 
-def _raw_main(args: list[str]) -> None:
+def _raw_main(args: list[str] | None) -> None:
     """A helper function for `main` without top-level exception
     handling.
     """
@@ -993,7 +993,7 @@ def parse_csl_callback(
     lib._close()
 
 
-def main(args: list[str]) -> None:
+def main(args: list[str] | None = None) -> None:
     """Run the main command-line interface for beets. Includes top-level
     exception handlers that print friendly error messages.
     """