Incorrect writing of NULL rows for struct column containing Decimal with precision >18 to Parquet

[nameexhaustion]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import io
import polars as pl

pl.DataFrame(
    {"x": [{"a": None}, None]}, schema={"x": pl.Struct({"a": pl.Decimal(19, 2)})}
).write_parquet(f := io.BytesIO())
f.seek(0)
print(pl.read_parquet(f))

# Output
# shape: (2, 1)
# ┌───────────┐
# │ x         │
# │ ---       │
# │ struct[1] │
# ╞═══════════╡
# │ null      │
# │ null      │
# └───────────┘
#
# Expected
# shape: (2, 1)
# ┌───────────┐
# │ x         │
# │ ---       │
# │ struct[1] │
# ╞═══════════╡
# │ {null}    │
# │ null      │
# └───────────┘

Log output

No response

Issue description

The outer validity gets incorrectly set.

Expected behavior

See example code

Installed versions

1.11.0