df.withColumn("Test", lit(null)).show()
+----+--------+-----+----+
|Hour|Category|Value|Test|
+----+--------+-----+----+
| 0| cat26| 30.9|null|
| 1| cat67| 28.5|null|
| 2| cat56| 39.6|null|
| 3| cat8| 35.6|null|
+----+--------+-----+----+