[zsh] Keep current $BUFFER on ALT-C

Ideally, we could only use `print -sr` to update the command history.
However, the "cd" command by ALT-C is added to the history only after we
finalize the current command by pressing an additional enter key.

i.e. The cd command from ALT-C is not visible when you hit Up arrow. But
it appears once you hit enter key.

So when the current buffer is empty, we use `zle accept-line` so that
the command history is immediately updated.

Close #2200