wrong default value type in Schema.__init__

[WH-2099]

Checks

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

Code

def __init__(
        self,
        schema: (
            Mapping[str, SchemaInitDataType]
            | Iterable[tuple[str, SchemaInitDataType]]
            | None
        ) = None,
        *,
        check_dtypes: bool = True,
    ) -> None:
        input = (
            schema.items() if schema and isinstance(schema, Mapping) else (schema or {})
        )
        for name, tp in input:  # type: ignore[misc]

Issue description

schma.py

The branch after else here corresponds to a schema parameter type of Iterable or None (strictly speaking, in the case of NoneType).
At the same time, the subsequent for statement indicates that the input here should be Iterable[tuple[str, SchemaInitDataType]].

Expected behavior

The default value after else should be () rather than {}.

Installed versions

--------Version info---------
Polars:              1.29.0
Index type:          UInt32
Platform:            Linux-6.14.4-arch1-1-x86_64-with-glibc2.41
Python:              3.13.3 (main, Apr  9 2025, 07:44:25) [GCC 14.2.1 20250207]
LTS CPU:             False

----Optional dependencies----
Azure CLI            <not installed>
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <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         0.17.0
matplotlib           <not installed>
numpy                2.2.5
openpyxl             <not installed>
pandas               <not installed>
polars_cloud         <not installed>
pyarrow              <not installed>
pydantic             2.11.4
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[WH-2099]

This is my first PR, please forgive me if there are any issues, I will follow up quickly