[MINOR] [SQL] Fix error message string in nullSafeEvel of TernaryExpression

## What changes were proposed in this pull request?

TernaryExpressions should thows proper error message for itself.
```scala
   protected def nullSafeEval(input1: Any, input2: Any, input3: Any): Any =
-    sys.error(s"BinaryExpressions must override either eval or nullSafeEval")
+    sys.error(s"TernaryExpressions must override either eval or nullSafeEval")
```

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12642 from dongjoon-hyun/minor_fix_error_msg_in_ternaryexpression.