Fix memory bloat from ComposerCollector

[staabm]

a collector should either return a valid result or null - because the returned value is persisted in PHPStan result cache (except for null). therefore you should not return things like [], as this would get serialized per AST node your collector works on.

thats something I learned today and in one of our projects it lead to out-of-memory fatal errors.

for in-detail context see phpstan/phpstan#11701 (comment)

[pb30]

Nice, thanks!