Fix if word-at-point is nil

`word-at-point` may return nil and it causes an error "Wrong type argument: arrayp, nil" in `erc-hl-nicks` (because `erc-hl-nicks-trim-irc-nick` is called with nil).

Sometimes I get that error when I want to send a message.  I'm not sure, but perhaps it happens because I've recently switched to:
```el
(setq erc-insert-timestamp-function 'erc-insert-timestamp-left)
```

This patch fixes my problem.