Zsh: check existing of file before expanding

echo ${test_var:-~/test(N)} 

comes close. By adding a (N) (and here as we are in list context), that ~/test(N) becomes a glob, and because of that N glob qualifier, if there's no test directory entry in $HOME (~), then the glob expands to nothing.

That's different from what you're asking in that it expands to nothing at all, instead of a one space argument.

For that, you could still do it in to steps:

f=(~/test(N)); echo ${test_var:-${f:-' '}} 

You could also use command substitution, though that's a bit cheating:

echo ${test_var:-"${$(printf %s ~/test(N)):-' '}"} 

Or:

echo ${test_var:-"$((){<<<${1-' '}} ~/test(N))"}