When following this pattern:
buildx build --arg FOO
Where we want to pull `FOO` from env, currently we always set `FOO`
regardless if the `FOO` env var is even set.
This change makes it so that `FOO` would only be set if it has been set
in the env (even if it is set to empty).
Signed-off-by: Brian Goff <cpuguy83@gmail.com>