do not try to convert null values in comparer

musify/processors/compare.py

@@ -118,7 +118,7 @@ def compare[T: Any](self, item: T, reference: T | None = None) -> bool:
 
     def _convert_expected(self, value: Any) -> None:
         """Driver for converting expected values to the same type as given value"""
-        if self._converted:
+        if self._converted or value is None:
             return
 
         if isinstance(value, int):