write_parquet does not preserve enum categories for empty data frames

[borchero]

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

For an empty data frame, writing to and reading from a parquet file turns enums into categoricals.

import polars as pl

df = pl.DataFrame(schema={"a": pl.Enum(["a", "b", "c"])})
df
# shape: (0, 1)
# ┌──────┐
# │ a    │
# │ ---  │
# │ enum │
# ╞══════╡
# └──────┘

df.write_parquet("out.parquet")
df_new = pl.read_parquet("out.parquet")
df_new
# shape: (0, 1)
# ┌─────┐
# │ a   │
# │ --- │
# │ cat │
# ╞═════╡
# └─────┘

df_new.schema
# Schema({'a': Categorical(ordering='physical')})

The same does not happen for non-empty data frames:

import polars as pl

df = pl.DataFrame({"a": ["a"]}, schema={"a": pl.Enum(["a", "b", "c"])})
df
# shape: (1, 1)
# ┌──────┐
# │ a    │
# │ ---  │
# │ enum │
# ╞══════╡
# │ a    │
# └──────┘

df.write_parquet("out.parquet")
df_new = pl.read_parquet("out.parquet")
df_new
# shape: (1, 1)
# ┌──────┐
# │ a    │
# │ ---  │
# │ enum │
# ╞══════╡
# │ a    │
# └──────┘

df_new.schema
# Schema({'a': Enum(categories=['a', 'b', 'c'])})

Log output

No response

Issue description

See above.

Expected behavior

The expected behavior would be to preserve the enum information even when empty data frames are written to parquet.

Installed versions

--------Version info---------
Polars:              1.16.0
Index type:          UInt32
Platform:            macOS-14.6.1-arm64-arm-64bit-Mach-O
Python:              3.13.0 | packaged by conda-forge | (main, Nov 27 2024, 19:18:26) [Clang 18.1.8 ]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                2.1.3
openpyxl             <not installed>
pandas               <not installed>
pyarrow              <not installed>
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>