Also modifies an existing integration test to expect an empty string as `docker-compose` warns that
`ZZVAR3` is not set and defaults it to an empty string per the acutal output here.
```yaml
$ docker-compose -f container-compose.load-.env-in-project.yaml config
WARN[0000] The "ZZVAR3" variable is not set. Defaulting to a blank string.
name: project
services:
app:
command:
- /bin/busybox
- sh
- -c
- env | grep ZZ
environment:
ZZVAR1: This value is loaded but should be overwritten
ZZVAR2: This value is loaded from .env in project/ directory
ZZVAR3: ""
...
```
Signed-off-by: indra <indra.talip@gmail.com>
