feat!: Update clip to no longer propagate nulls in the given bounds

[stinodego]

Closes #13714

Changes

• Null values in the bounds no longer set the value to null - instead, the original value is retained.

Before

>>> df = pl.DataFrame({"a": [0, 1, 2], "min": [1, None, 1]})
>>> df.select(pl.col("a").clip("min"))
shape: (3, 1)
┌──────┐
│ a    │
│ ---  │
│ i64  │
╞══════╡
│ 1    │
│ null │
│ 2    │
└──────┘

After

>>> df.select(pl.col("a").clip("min"))
shape: (3, 1)
┌──────┐
│ a    │
│ ---  │
│ i64  │
╞══════╡
│ 1    │
│ 1    │
│ 2    │
└──────┘