zsh: Return 0 from command_not_found_handler

"The function should return status zero if it successfully handled
the command, or non-zero status if it failed. In the latter case
the standard handling is applied: ‘command not found’ is printed
to standard error and the shell exits with status 127."
- http://zsh.sourceforge.net/Doc/Release/Command-Execution.html

Needed after changes in
https://github.com/termux/termux-packages/pull/566
where the command-not-found program no longer exists itself with 0.