[SPARK-15709][SQL] Prevent `freqItems` from raising `UnsupportedOperationException: empty.min`

## What changes were proposed in this pull request?

Currently, `freqItems` raises `UnsupportedOperationException` on `empty.min` usually when its `support` argument is high.
```scala
scala> spark.createDataset(Seq(1, 2, 2, 3, 3, 3)).stat.freqItems(Seq("value"), 2)
16/06/01 11:11:38 ERROR Executor: Exception in task 5.0 in stage 0.0 (TID 5)
java.lang.UnsupportedOperationException: empty.min
...
```

Also, the parameter checking message is wrong.
```
require(support >= 1e-4, s"support ($support) must be greater than 1e-4.")
```

This PR changes the logic to handle the `empty` case and also improves parameter checking.

## How was this patch tested?

Pass the Jenkins tests (with a new testcase).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13449 from dongjoon-hyun/SPARK-15709.