polars.exceptions.ColumnNotFoundError when coerce=True and Optional field is missing

[antonioalegria]

Describe the bug
A clear and concise description of what the bug is.

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera.
• (optional) I have confirmed this bug exists on the main branch of pandera.

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Code Sample, a copy-pastable example

from pandera.polars import Field # type: ignore
from pandera.polars import DataFrameModel # type: ignore

from typing import Optional

import polars as pl


class MyModel(DataFrameModel):
    a: Optional[str] = Field(description="some description", nullable=True)
    b: Optional[str] = Field(description="some description") # BOOM
    c: Optional[str] = Field(description="some description", str_contains=".", nullable=True)
    d: Optional[str] = Field(description="some description", str_contains=".") # BOOM

df = pl.DataFrame({})
schema = MyModel.to_schema()
schema.strict = True
schema.coerce = True # -> without this it works
print(schema.validate(df)) # BOOM

Exception:
(.venv) antonioalegria@shiro dojo % /Users/antonioalegria/Developer/dojo/.venv/bin/python /Users/antonioalegria/Developer/dojo/dojo/test.py
Traceback (most recent call last):
File ".../test.py", line 22, in
print(schema.validate(df))
^^^^^^^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pandera/api/polars/container.py", line 58, in validate
output = self.get_backend(check_obj).validate(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pandera/backends/polars/container.py", line 63, in validate
check_obj = parser(check_obj, *args)
^^^^^^^^^^^^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pandera/backends/polars/container.py", line 396, in coerce_dtype
check_obj = self._coerce_dtype_helper(check_obj, schema)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pandera/backends/polars/container.py", line 455, in _coerce_dtype_helper
obj = getattr(col_schema.dtype, coerce_fn)(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../.venv/lib/python3.12/site-packages/pandera/engines/polars_engine.py", line 181, in try_coerce
lf.collect()
File ".../.venv/lib/python3.12/site-packages/polars/lazyframe/frame.py", line 2034, in collect
return wrap_df(ldf.collect(callback))
^^^^^^^^^^^^^^^^^^^^^
polars.exceptions.ColumnNotFoundError: a

Expected behavior

The dataframe should've been validated.

Desktop (please complete the following information):

OS: macOS 14.6.1
Python 3.12.4
polars-lts-cpu 1.6.0
pandera 0.20.3

Screenshots

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Additional context

Add any other context about the problem here.

[IsaiasGutierrezCruz]

I have the same problem. If the schema does not coerce the types, the Optional type works as expected (the DataFrame does not require that column). Otherwise, the schema requires that column.