[vim/vim] Vim9: listener_add() fails when given only a callback (PR #21116)

```
Problem:  In Vim9 script, listener_add() given only a callback fails
          with "E1212: Bool required for argument 3".
Solution: Check the third argument only when the second one is there.
          Without a second argument the third is not filled in, so
          what was checked was whatever happened to be in that slot.
```
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21116

-- Commit Summary --

  * Vim9: listener_add() fails when given only a callback

-- File Changes --

    M src/change.c (3)
    M src/testdir/test_listener.vim (24)

-- Patch Links --

https://github.com/vim/vim/pull/21116.patch
https://github.com/vim/vim/pull/21116.diff

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