fix: initialize and pass envFiles map in processExtends

[Mohamed-Moumni]

Title: fix: initialize envFiles map in processExtends to handle env_file during publish

Description:

What I did

Initialize envFiles as an empty map in processExtends and pass it to processFile instead of nil.

Why

When running docker compose publish on a project that uses extends with services referencing env_file, processFile attempts to assign entries into the envFiles map. Since nil was passed, this caused a panic:

panic: assignment to entry in nil map
goroutine 1 [running]:
github.com/docker/compose/v5/pkg/compose.processFile(...)
    github.com/docker/compose/v5/pkg/compose/publish.go:255 +0x651

Fixes #13671

[glours]

LGTM, thanks for the contribution 🙏

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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