[SPARK-16006][SQL] Attemping to write empty DataFrame with no fields throws non-intuitive exception

## What changes were proposed in this pull request?

This PR allows `emptyDataFrame.write` since the user didn't specify any partition columns.

**Before**
```scala
scala> spark.emptyDataFrame.write.parquet("/tmp/t1")
org.apache.spark.sql.AnalysisException: Cannot use all columns for partition columns;
scala> spark.emptyDataFrame.write.csv("/tmp/t1")
org.apache.spark.sql.AnalysisException: Cannot use all columns for partition columns;
```

After this PR, there occurs no exceptions and the created directory has only one file, `_SUCCESS`, as expected.

## How was this patch tested?

Pass the Jenkins tests including updated test cases.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13730 from dongjoon-hyun/SPARK-16006.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningUtils.scala

@@ -351,7 +351,7 @@ private[sql] object PartitioningUtils {
       }
     }
 
-    if (partitionColumns.size == schema.fields.size) {
+    if (partitionColumns.nonEmpty && partitionColumns.size == schema.fields.length) {
       throw new AnalysisException(s"Cannot use all columns for partition columns")
     }
   }

sql/core/src/test/scala/org/apache/spark/sql/test/DataFrameReaderWriterSuite.scala

@@ -246,8 +246,9 @@ class DataFrameReaderWriterSuite extends QueryTest with SharedSQLContext with Be
         spark.range(10).write.format("parquet").mode("overwrite").partitionBy("id").save(path)
       }
       intercept[AnalysisException] {
-        spark.range(10).write.format("orc").mode("overwrite").partitionBy("id").save(path)
+        spark.range(10).write.format("csv").mode("overwrite").partitionBy("id").save(path)
       }
+      spark.emptyDataFrame.write.format("parquet").mode("overwrite").save(path)
     }
   }