test precmd_function without test compound command and command substitution

[d630]

Tested with:

% bash --version
GNU bash, version 4.3.42(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[d630]

I prefer if declare -F function 1>/dev/null, but we can also use if function

[rcaloras]

Either one works for me. Feel free to submit another pull with that if you'd like.

[rcaloras]

👍 LGTM 🍔 🍟