bug #1083 Fix "null as array offset" deprecation in DockerComposeConfigurator (GromNaN)

This PR was merged into the 2.x branch.

Discussion
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Fix "null as array offset" deprecation in DockerComposeConfigurator

Fix #1079

## Summary

PHP 8.5 introduces a deprecation notice when `null` is used as an array offset:

> Using null as an array offset is deprecated, use an empty string instead

In `configureDockerCompose()`, `$node` is initialized to `null` and used as an array key in `$endAt[$node]` before being assigned — once at the first top-level key encounter during the parsing loop, and once after the loop when the file is empty (e.g. a freshly created `compose.yaml`).

## Fix

Guard both `$endAt[$node]` assignments with `null !== $node` checks, since there is no previous section to record an end position for.

Commits
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69e556f Fix "null as array offset" deprecation in DockerComposeConfigurator

Author: nicolas-grekas