Visit catch even when its body is empty (#1141)
The current visitor does not visit `catch` itself when its body is
empty. So even if the attached test with an empty `catch` body is
supposed to be a validation failure (because an `exnref` value is left
on the stack in the `catch` body), it passed the validator.
In more detail, in the current code,
Input: (should be a validation failure)
```
try
catch ;; this pushes an exnref value onto the stack
end
```
Output: (wrong, `catch` is missing)
```
try
end
```
Now we correctly visit `catch` even when its body is empty, this test
correctly causes a validation failure.
