In the interests of promoting creativity, I'll ask the question without
any prior assumptions:

I'm using popen4 (great gem, btw) to run an external command and
capture stdout and stderr.  If one (or both) of those is empty, I don't
want to hang.  Just keep on moving.  In fact, stderr will normally be
empty.

How could I do this?

(I'm supporting 'nix and Windows)