fix: Return null instead of 0. for rolling_std when window contains a single element and ddof=1 and there are nulls elsewhere in the Series

[MarcoGorelli]

closes #20076

[MarcoGorelli]

like this it matches what's done for the no_nulls/variance.rs kernel

polars/crates/polars-arrow/src/legacy/kernels/rolling/no_nulls/variance.rs

Lines 121 to 125 in b618734

	let denom = count - NumCast::from(self.ddof).unwrap();
	if denom <= T::zero() {
	None
	} else if end - start == 1 {
	Some(T::zero())

[MarcoGorelli]

here the original series is [1, null, -1, 4], and the operation is rolling variance with window of 3 and min_periods 1 and ddof=1

So the windows are:

• [1]
• [1, null]
• [1, null, -1]
• [null, -1, 4]

and so it's correct that the first two need to output None, as they only have a single valid value and ddof=1:

In [50]: print(pl.Series([1]).std())
None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.53%. Comparing base (954000c) to head (1afd302).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20077   +/-   ##
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  Coverage   79.52%   79.53%           
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  Files        1563     1563           
  Lines      217124   217115    -9     
  Branches     2464     2464           
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+ Hits       172674   172677    +3     
+ Misses      43890    43878   -12     
  Partials      560      560           

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