## What changes were proposed in this pull request?
Below are the two cases.
``` SQL
case 1
scala> List.empty[String].toDF().rdd.partitions.length
res18: Int = 1
```
When we write the above data frame as parquet, we create a parquet file containing
just the schema of the data frame.
Case 2
``` SQL
scala> val anySchema = StructType(StructField("anyName", StringType, nullable = false) :: Nil)
anySchema: org.apache.spark.sql.types.StructType = StructType(StructField(anyName,StringType,false))
scala> spark.read.schema(anySchema).csv("/tmp/empty_folder").rdd.partitions.length
res22: Int = 0
```
For the 2nd case, since number of partitions = 0, we don't call the write task (the task has logic to create the empty metadata only parquet file)
The fix is to create a dummy single partition RDD and set up the write task based on it to ensure
the metadata-only file.
## How was this patch tested?
A new test is added to DataframeReaderWriterSuite.
Author: Dilip Biswal <dbiswal@us.ibm.com>
Closes#20525 from dilipbiswal/spark-23271.
d90e77bd0e