[SPARK-43230][CONNECT] Simplify DataFrameNaFunctions.fillna

[zhengruifeng]

What changes were proposed in this pull request?

add a helper function in DataFrameNaFunctions

Why are the changes needed?

to Simplify DataFrameNaFunctions.fillna

Does this PR introduce any user-facing change?

no, dev-only

How was this patch tested?

existing UTs

[amaliujia]

LGTM

[zhengruifeng]

ERROR: Comparing client jar: /__w/spark/spark/connector/connect/client/jvm/target/scala-2.12/spark-connect-client-jvm-assembly-3.5.0-SNAPSHOT.jar and and sql jar: /__w/spark/spark/sql/core/target/scala-2.12/spark-sql_2.12-3.5.0-SNAPSHOT.jar 
problems: 
method fillValue(java.lang.Object,scala.Option)org.apache.spark.sql.Dataset in class org.apache.spark.sql.DataFrameNaFunctions does not have a correspondent in client version
Exceptions to binary compatibility can be added in 'CheckConnectJvmClientCompatibility#checkMiMaCompatibility'
connect-client-jvm module mima check failed.
Error: Process completed with exit code 1.

seems Mima also check private[sql]

[zhengruifeng]

@LuciferYang it seems the MiMa tests also check private[sql], is this as expected?

[LuciferYang]

This behavior is expected now, but I think some refactoring work is needed to automatically filter similar checks, just like what dev/mima check does.

[zhengruifeng]

got it, thanks

[LuciferYang]

filed https://issues.apache.org/jira/browse/SPARK-43246 to tracking this

[LuciferYang]

Also cc @zhenlineo, @hvanhovell do you have any good suggestions about this

[LuciferYang]

#40925 try to solve this issue, but it will make CheckConnectJvmClientCompatibility spend more time and maybe more complex

[zhengruifeng]

Thanks for the reviews, merged to master