[BUG] False "variable is not set" warning when using the ${A:+} interpolation syntax

Description

When a variable interpolation is nested inside a ${A:+}-style interpolation construct for the same variable, compose emits a "variable is not set" warning, even though the nested evaluation isn't supposed to be evaluated.

Steps To Reproduce

Create compose.yml as follows:

services:
  test:
    environment:
      A: ${A:+${A},}B
    image: busybox
    command: ['sh', '-c', 'echo "$$A"']

Run docker compose up. It prints the following:

WARN[0000] The "A" variable is not set. Defaulting to a blank string.
[+] Running 1/1
 ✔ Container docker-compose-repro-test-1  Recreated                                                                0.1s
Attaching to test-1
test-1  | B
test-1 exited with code 0

The warning is a false positive: the nested ${A}, expression should only be evaluated if A is defined, which it isn't.

Compose Version

Docker Compose version v2.33.1

Docker Environment

Client: Docker Engine - Community
 Version:    28.0.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.21.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.33.1
    Path:     /usr/libexec/docker/cli-plugins/docker-compose
  scout: Docker Scout (Docker Inc.)
    Version:  v1.14.0
    Path:     /home/dpb/.docker/cli-plugins/docker-scout

Server:
 Containers: 3
  Running: 1
  Paused: 0
  Stopped: 2
 Images: 242
 Server Version: 28.0.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: bcc810d6b9066471b0b6fa75f557a15a1cbf31bb
 runc version: v1.2.4-0-g6c52b3f
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
 Kernel Version: 5.15.133.1-microsoft-standard-WSL2
 Operating System: Ubuntu 24.04.2 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 12
 Total Memory: 11.68GiB
 Name: [redacted]
 ID: ded0336a-e1e9-4025-8553-4b4392a16670
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No blkio throttle.read_bps_device support
WARNING: No blkio throttle.write_bps_device support
WARNING: No blkio throttle.read_iops_device support
WARNING: No blkio throttle.write_iops_device support

Anything else?

For comparison, in bash an equivalent scenario works as expected:

$ bash -u -c 'echo ${A:+${A},}B'
B

The -u option prohibits interpolating undefined variables, but no error is produced.