keep empty variables in service.environment list

[glours]

those variables could be used to unset variables in the container

The issue was detected by Docker Compose CI
https://github.com/docker/compose/actions/runs/14223750088/job/39858067378?pr=12692

=== Failed
=== FAIL: pkg/e2e TestUnsetEnv/unset_env_variable (0.71s)
    compose_environment_test.go:241: Running command: docker compose -f ./fixtures/environment/empty-variable/compose.yaml run --rm empty-variable
    compose_environment_test.go:243: assertion failed: 
        Command:  docker compose -f ./fixtures/environment/empty-variable/compose.yaml run --rm empty-variable
        ExitCode: 0
        Stdout:   =not_empty=
        
        Stderr:   
        
        Failures:
        Expected stdout to contain "=="
    --- FAIL: TestUnsetEnv/unset_env_variable (0.71s)