### What changes were proposed in this pull request?
#26700 removed the ability to drop a row whose nested column value is null.
For example, for the following `df`:
```
val schema = new StructType()
.add("c1", new StructType()
.add("c1-1", StringType)
.add("c1-2", StringType))
val data = Seq(Row(Row(null, "a2")), Row(Row("b1", "b2")), Row(null))
val df = spark.createDataFrame(spark.sparkContext.parallelize(data), schema)
df.show
+--------+
| c1|
+--------+
| [, a2]|
|[b1, b2]|
| null|
+--------+
```
In Spark 2.4.4,
```
df.na.drop("any", Seq("c1.c1-1")).show
+--------+
| c1|
+--------+
|[b1, b2]|
+--------+
```
In Spark 2.4.5 or Spark 3.0.0-preview2, if nested columns are specified, they are ignored.
```
df.na.drop("any", Seq("c1.c1-1")).show
+--------+
| c1|
+--------+
| [, a2]|
|[b1, b2]|
| null|
+--------+
```
### Why are the changes needed?
This seems like a regression.
### Does this PR introduce any user-facing change?
Now, the nested column can be specified:
```
df.na.drop("any", Seq("c1.c1-1")).show
+--------+
| c1|
+--------+
|[b1, b2]|
+--------+
```
Also, if `*` is specified as a column, it will throw an `AnalysisException` that `*` cannot be resolved, which was the behavior in 2.4.4. Currently, in master, it has no effect.
### How was this patch tested?
Updated existing tests.
Closes#28266 from imback82/SPARK-31256.
Authored-by: Terry Kim <yuminkim@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
d7499aed9c