dockerfile: Add integration test for ARG with empty value and default

[tonistiigi]

Follow-up for #4897 (comment)

Seems that we are missing a test that would check that ARG without default value would not be defined in the environment variables of RUN steps at all (instead of showing up with empty value).

In a Dockerfile like

FROM alpine
ARG FOO
ARG BAR=
RUN env && stop

The env output should show only BAR= and no FOO= .