[BUG] Scala Test md5 can produce non-empty nulls (merge and set validity)

[revans2]

Describe the bug
To be able to support sorting and comparisons on nested types we needed to get rid of all non-empty nulls. To help with this we added in some asserts when we see them. It looks like the GpuMd5 implementation is using mergeAndSetValidity which is producing an exception in this case.

Steps/Code to reproduce bug
Run the unit tests, but revert #8183 first

 org.apache.spark.SparkException: Job aborted due to stage failure: Task 0 in stage 845.0 failed 1 times, most recent failure: Lost task 0.0 in stage 845.0 (TID 2577) (10.28.9.123 executor driver): java.lang.AssertionError: Column has non-empty nulls
	at ai.rapids.cudf.AssertEmptyNulls.assertNullsAreEmpty(AssertEmptyNulls.java:33)
	at ai.rapids.cudf.ColumnView.<init>(ColumnView.java:71)
	at ai.rapids.cudf.ColumnVector.<init>(ColumnVector.java:58)
	at ai.rapids.cudf.ColumnView.mergeAndSetValidity(ColumnView.java:835)
	at org.apache.spark.sql.rapids.GpuMd5.$anonfun$doColumnar$2(HashFunctions.scala:38)
	at com.nvidia.spark.rapids.Arm$.withResource(Arm.scala:29)
	at org.apache.spark.sql.rapids.GpuMd5.$anonfun$doColumnar$1(HashFunctions.scala:37)
	at com.nvidia.spark.rapids.Arm$.withResource(Arm.scala:29)
	at org.apache.spark.sql.rapids.GpuMd5.doColumnar(HashFunctions.scala:36)

Expected behavior
This should not happen, we either need to fix mergeAndSetValidity, which I thought we had done, but it might have been bitwiseMergeAndSetValidity. Not sure if we have two or not. Or we need to stop using it + #7698

[razajafri]

fixed by rapidsai/cudf#13335