Bug: Roundtrip a collection with parquet fails with `dy.Any`

[gab23r]

dy.Any columns should be ignored in the validation when reading from a parquet file.

from pathlib import Path
import dataframely as dy
import polars as pl

class S(dy.Schema):
    a = dy.Any()

class C(dy.Collection):
    df : dy.LazyFrame[S]

df = pl.DataFrame({"a": 0})
collection = C.validate({"df": df})


Path("./tmp").mkdir(exist_ok=True)
collection.write_parquet("./tmp")
collection.read_parquet("./tmp").df.collect()

# InvalidOperationError: conversion from `i64` to `null` failed in column 'a' for 1 out of 1 values: [0]

# This error occurred in the following expression:
# 	col("a").strict_cast(Null)

[gab23r]

after a bit of investigation the root cause is that cast try to map dy.Any to pl.Null:

S.validate(df) # works
S.cast(df) # InvalidOperationError: conversion from `i64` to `null` failed in column 'a' for 1 out of 1 values: [0]