Panic when doing an inner join on two empty DFs

[max-muoto]

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({"col1": [], "col2": [], "col3": []})
df2 = pl.DataFrame({"col2": [], "col4": [], "col5": []})

df1.join(df2, on="col2", how="inner")

Issue description

When trying to do an inner join on two empty DFs, you'll run into a panic. Personally would expect an empty DF with all columns to be returned. Here's a quick example in Colab: https://colab.research.google.com/drive/1q8HS7oX7f-LZzgbvnuGDeG8ZbPq0wj43?usp=sharing. However I also tested this locally on 0.20.25.

Expected behavior

import polars as pl
df1 = pl.DataFrame({"col1": [], "col2": [], "col3": []})
df2 = pl.DataFrame({"col2": [], "col4": [], "col5": []})
resulting_df = df1.join(df2, on="col2", how="inner")

In this case, resulting_df would be an empty DF with col1, col2, col3, col4, and col5.

Installed versions

--------Version info---------
Polars:               0.20.25
Index type:           UInt32
Platform:             macOS-14.4.1-arm64-arm-64bit
Python:               3.11.7 | packaged by conda-forge | (main, Dec 23 2023, 14:38:07) [Clang 16.0.6 ]

----Optional dependencies----
adbc_driver_manager:  0.11.0
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         1.6.0
numpy:                1.26.1
openpyxl:             3.1.2
pandas:               2.2.0
pyarrow:              14.0.2
pydantic:             2.7.1
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             0.8.2
xlsxwriter:           3.1.9```

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