zshrc: using FOO=${FOO:-default} instead of (( ${+FOO} )) || FOO=default

zshrc: using FOO=${FOO:-default} instead of (( ${+FOO} )) || FOO=default

This is more correct, IMHO.
The old way only sets the default if $FOO is unset; whereas the new way
sets the default if $FOO is unset *or* empty (which is certainly desirable,
if you think of variables like $PAGER).