chore: deduplicate filter nulls code in coalesce/filter kernel

[Jefffrey]

whilst reviewing #10236 i noticed we have a little code duplication, so remedying that

[Jefffrey]

since filter_nulls() already creates the nullbuffer for us, just use that

arrow-rs/arrow-select/src/filter.rs

Lines 490 to 517 in a4ac705

	/// Filters the given `nulls` buffer using this predicate.
	///
	/// Returns `None` when there is nothing to track in the output, either
	/// because the input `nulls` was `None`, the input had no nulls, or the
	/// filtered result has no nulls. Otherwise returns the filtered
	/// [`NullBuffer`] with its precomputed null count.
	pub fn filter_nulls(&self, nulls: Option<&NullBuffer>) -> Option<NullBuffer> {
	let nulls = nulls?;
	if nulls.null_count() == 0 {
	return None;
	}
	let nulls = filter_bits(nulls.inner(), self);
	// The filtered `nulls` has a length of `self.count` bits and therefore
	// the null count is this minus the number of valid bits
	let null_count = self.count - nulls.count_set_bits_offset(0, self.count);
	if null_count == 0 {
	return None;
	}
	let buffer = BooleanBuffer::new(nulls, 0, self.count);
	debug_assert_eq!(null_count, buffer.len() - buffer.count_set_bits());
	// SAFETY: `null_count` was derived from `buffer` above, so it matches
	// the number of unset bits as required by `new_unchecked`.
	Some(unsafe { NullBuffer::new_unchecked(buffer, null_count) })
	}
	}