Projection pushdown on scalars changes output height

[nameexhaustion]

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

q = pl.LazyFrame({"a": range(5)}).select("a", b=pl.col("a").first()).select("b")
print(q.collect())
# shape: (1, 1)
# ┌─────┐
# │ b   │
# │ --- │
# │ i64 │
# ╞═════╡
# │ 0   │
# └─────┘
print(q.collect(projection_pushdown=False))
# shape: (5, 1)
# ┌─────┐
# │ b   │
# │ --- │
# │ i64 │
# ╞═════╡
# │ 0   │
# │ 0   │
# │ 0   │
# │ 0   │
# │ 0   │
# └─────┘

Log output

No response

Issue description

Result becomes 1 row instead instead of 5 rows with projection pushdown

Expected behavior

Maintains 5 rows

Installed versions

1.17.0