Fix dh-exec-subst-env handling of empty vars

When an environment variable is set to an empty variable or 0, dh-exec ignores
it and restores the ${foo} placeholder, but this shouldn't be the case.

This commit checks for the existence of the variable in %ENV instead of just
using || for falling back.
tests: Add a test case for empty value substitution

This adds a test case for the previous commit.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Release version 0.8

Bump the version in configure.ac, and add the news to debian/changelog.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>