Multiplication by const raises `(left == right)` assert exception

[telperions]

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Issue description

When filtering a colum in agg expression and after filtering we have 0 values, diff + multiplication may raise

pyo3_runtime.PanicException: assertion failed: `(left == right)`
 left: `0`,
right: `1`.

Note that if we moved * 1000 all works as expected:

pl.DataFrame({'col_to_groupby': [2], 'time': [1672740910.967138],  'col3': [1]}).groupby('col_to_groupby').agg(
   (pl.col('time').filter(pl.col('col3') == 0).diff().fill_null(strategy='zero')
   .diff().fill_null(strategy='zero') * 1000).mean().alias('fwd_iat_ho_mean').alias('calc')
)

Reproducible example

import polars as pl

pl.DataFrame({'col_to_groupby': [2], 'time': [1672740910.967138],  'col3': [1]}).groupby('col_to_groupby').agg(
    (pl.col('time').filter(pl.col('col3') == 0).diff() * 1000).fill_null(strategy='zero')
    .diff().fill_null(strategy='zero').mean().alias('calc')
)
pyo3_runtime.PanicException: assertion failed: `(left == right)`
  left: `0`,
 right: `1`

Expected behavior

Computation should return:

col_to_groupby	calc
i64	f64
2	null

Installed versions

---Version info---
Polars: 0.15.11
Index type: UInt32
Platform: Linux-5.15.0-56-generic-x86_64-with-glibc2.35
Python: 3.8.16 (default, Dec  7 2022, 01:12:06) 
[GCC 11.3.0]
---Optional dependencies---
pyarrow: 10.0.1
pandas: <not installed>
numpy: 1.22.3
fsspec: 2022.8.2
connectorx: <not installed>
xlsx2csv: <not installed>
matplotlib: <not installed>
None