Running tests triggers deprecations on PHP 8.5

[stof]

Running tests on PHP 8.5 triggers many deprecations Using null as an array offset is deprecated, use an empty string instead (as reported in https://github.com/async-aws/aws/actions/runs/18214863956/job/51861965683#step:9:52)

Those deprecations should be investigated to see where they happen, to allow fixing them (either in core or by improving the code generator depending on their location)