[SPARK-40579][PS] GroupBy.first should skip NULLs

[zhengruifeng]

What changes were proposed in this pull request?

make GroupBy.first skip nulls

Why are the changes needed?

to fix the behavior difference

In [1]: 
   ...: import pandas as pd
   ...: import numpy as np
   ...: import pyspark.pandas as ps
   ...: 
   ...: pdf = pd.DataFrame({"A": [1, 2, 1, 2],"B": [-1.5, np.nan, -3.2, 0.1],})
   ...: psdf = ps.from_pandas(pdf)
   ...: 

In [2]: pdf.groupby("A").first()
Out[2]: 
     B
A     
1 -1.5
2  0.1

In [3]: psdf.groupby("A").first()
                                                                                
     B
A     
1 -1.5
2  NaN

Does this PR introduce any user-facing change?

yes, updated GroupBy.first will skip NULLs

How was this patch tested?

added UT

[itholic]

Good catch!

[dongjoon-hyun]

+1, LGTM.

[HyukjinKwon]

Merged to master.

[zhengruifeng]

Thanks you @HyukjinKwon @dongjoon-hyun @itholic