entered unreachable code when using collect_schema and concat

[da2225]

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

empty = pl.LazyFrame()
empty.collect_schema()
pl.concat([empty, empty]).collect()

Log output

found multiple sources; run comm_subplan_elim
thread '<unnamed>' panicked at crates/polars-mem-engine/src/planner/lp.rs:678:20:
internal error: entered unreachable code
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Issue description

In the example code, both the re-use of the 'empty' dataframe and the call to collect_schema seem to be needed to break the collect call.

Expected behavior

Without the call to collect_schema, the code works as expected. It should work the same way with the call to collect_schema in place.

Installed versions

>>> pl.show_versions()
--------Version info---------
Polars:               1.5.0
Index type:           UInt32
Platform:             Linux-6.1.84-99.169.amzn2023.x86_64-x86_64-with-glibc2.34
Python:               3.12.5 | packaged by conda-forge | (main, Aug  8 2024, 18:36:51) [GCC 12.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                <not installed>
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>

[deanm0000]

This errors even if not empty for example

lf=pl.LazyFrame(dict(a=1))
lf.collect_schema()
pl.concat([lf, lf]).collect()