join_nulls has no effect when how="semi"

[etiennebacher]

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 polars as pl

df1 = pl.DataFrame({"a": [1, None], "val": [1, 2]})
df2 = pl.DataFrame({"a": [1, None], "val2": [3, 4]})

df1.join(df2, on="a", how="semi", join_nulls=False)
df1.join(df2, on="a", how="semi", join_nulls=True)

Log output

>>> df1.join(df2, on="a", how="semi", join_nulls=False)
shape: (2, 2)
┌──────┬─────┐
│ a    ┆ val │
│ ---  ┆ --- │
│ i64  ┆ i64 │
╞══════╪═════╡
│ 1    ┆ 1   │
│ null ┆ 2   │
└──────┴─────┘
>>> df1.join(df2, on="a", how="semi", join_nulls=True)
shape: (2, 2)
┌──────┬─────┐
│ a    ┆ val │
│ ---  ┆ --- │
│ i64  ┆ i64 │
╞══════╪═════╡
│ 1    ┆ 1   │
│ null ┆ 2   │
└──────┴─────┘

Issue description

When doing a semi-join, rows in df1 that don't have a match in df2 should be dropped. If join_nulls=False, then null keys in df1 don't have a match and therefore their rows should be dropped. Currently, join_nulls has no effect on the output of semi-joins.

Expected behavior

>>> df1.join(df2, on="a", how="semi", join_nulls=False)
shape: (2, 2)
┌──────┬─────┐
│ a    ┆ val │
│ ---  ┆ --- │
│ i64  ┆ i64 │
╞══════╪═════╡
│ 1    ┆ 1   │
└──────┴─────┘

Installed versions

--------Version info---------
Polars:               0.20.21
Index type:           UInt32
Platform:             Windows-10-10.0.19045-SP0
Python:               3.11.0 (main, Oct 24 2022, 18:26:48) [MSC v.1933 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2023.6.0
gevent:               <not installed>
hvplot:               0.9.2
matplotlib:           3.7.1
nest_asyncio:         1.5.6
numpy:                1.24.3
openpyxl:             <not installed>
pandas:               2.0.3
pyarrow:              12.0.1
pydantic:             2.6.4
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>