Fix `TypeError: issubclass() arg 1 must be a class` for optional list of bytes

fastapi/_compat/v2.py

@@ -371,6 +371,13 @@ def copy_field_info(*, field_info: FieldInfo, annotation: Any) -> FieldInfo:
 
 def serialize_sequence_value(*, field: ModelField, value: Any) -> Sequence[Any]:
     origin_type = get_origin(field.field_info.annotation) or field.field_info.annotation
+    if origin_type is Union:  # Handle optional sequences
+        union_args = get_args(field.field_info.annotation)
+        for union_arg in union_args:
+            if union_arg is type(None):
+                continue
+            origin_type = get_origin(union_arg) or union_arg
+            break
     assert issubclass(origin_type, shared.sequence_types)  # type: ignore[arg-type]
     return shared.sequence_annotation_to_type[origin_type](value)  # type: ignore[no-any-return]