### What changes were proposed in this pull request?
Modify the approach in `DataFrameNaFunctions.fillValue`, the new one uses `df.withColumns` which only address the columns need to be filled. After this change, there are no more ambiguous fileds detected for joined dataframe.
### Why are the changes needed?
Before this change, when you have a joined table that has the same field name from both original table, fillna will fail even if you specify a subset that does not include the 'ambiguous' fields.
```
scala> val df1 = Seq(("f1-1", "f2", null), ("f1-2", null, null), ("f1-3", "f2", "f3-1"), ("f1-4", "f2", "f3-1")).toDF("f1", "f2", "f3")
scala> val df2 = Seq(("f1-1", null, null), ("f1-2", "f2", null), ("f1-3", "f2", "f4-1")).toDF("f1", "f2", "f4")
scala> val df_join = df1.alias("df1").join(df2.alias("df2"), Seq("f1"), joinType="left_outer")
scala> df_join.na.fill("", cols=Seq("f4"))
org.apache.spark.sql.AnalysisException: Reference 'f2' is ambiguous, could be: df1.f2, df2.f2.;
```
### Does this PR introduce any user-facing change?
Yes, fillna operation will pass and give the right answer for a joined table.
### How was this patch tested?
Local test and newly added UT.
Closes#25768 from xuanyuanking/SPARK-29063.
Lead-authored-by: Yuanjian Li <xyliyuanjian@gmail.com>
Co-authored-by: Xiao Li <gatorsmile@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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