FunctionParamsProvider getFunctionParams() nullable but leads to type overwritten

[kkmuffme]

if a plugin implements getFunctionParams but returns null, it still overwrites any existing params already inferred

This is inconsistent compared to the ReturnTypeProvider and does not allow fall back to the default value

[psalm-github-bot]

Hey @kkmuffme, can you reproduce the issue on https://psalm.dev? These will be used as phpunit tests when implementing the feature or fixing this bug.