[BUG][JNI] java.lang.IllegalArgumentException: Unknown scalar type: STRUCT

**Describe the bug**
cuDF Java supports creating Scalar types to represent structs, but printing them for debug purposes causes an exception.

```
java.lang.IllegalArgumentException: Unknown scalar type: STRUCT
```

**Steps/Code to reproduce bug**
Create a scalar using `Scalar.structFromNull` or `Scalar.structFromColumnViews` and then call its `toString` method or try and print it using `System.out.println`

**Expected behavior**
Should print some useful debug info as per other scalar types, rather than throw an exception.

**Environment overview (please complete the following information)**
N/A

**Environment details**

**Additional context**