stream: Transform should allow falsey values

I've backported this commit from master branch(26ca7d73,
which was authored by Jeff Barczewski <jeff.barczewski@gmail.com>)
below is his commit message about why and how to fix this issue.

If a transform stream has objectMode = true, it should
allow falsey values other than (null) like 0, false, ''.

null is reserved to indicate stream eof but
other falsey values should flow through properly.