feat!: Preserve nulls in ewm_mean, ewm_std, and ewm_var

[MarcoGorelli]

closes #15258

This is a breaking change, so should wait for 1.0

I discussed it Ritchie and others at PyCon Lithuania, and people think this change makes sense. If a user wants their null values filled in, they can just .forward_fill - but it doesn't seem correct to do so on their behalf

Example

Before:

>>> s = pl.Series([1, 4, None, 3])
>>> s.ewm_mean(alpha=.9, ignore_nulls=False)
shape: (4,)
Series: '' [f64]
[
        1.0
        3.727273
        3.727273
        3.007913
]

After:

>>> s.ewm_mean(alpha=.9, ignore_nulls=False)
shape: (4,)
Series: '' [f64]
[
        1.0
        3.727273
        null
        3.007913
]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.14%. Comparing base (31df06d) to head (0fd9fcb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15503   +/-   ##
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  Coverage   81.14%   81.14%           
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  Files        1362     1362           
  Lines      174951   174953    +2     
  Branches     2533     2533           
=======================================
+ Hits       141960   141970   +10     
+ Misses      32508    32500    -8     
  Partials      483      483           

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