feat: infer nullable type information by filled/blank function calls

[tpetry]

The result value of the blank and filled helpers imply some type information that is currently not supported by Larastan. To be precise, the blank helper will return true on null while the filled will return false. This enhanced type inference is especially usefull when working with nullable Eloquent attributes:

if (filled($model->value)) {
  executeSomething($model->value);
}

Currently, PHPStan will report errors as null can't be passed to executeSomething. But the value can never be null because of a true result of the filled check. The new stubs help PHPStan to narrow the types of values passed to filled and blank functions.

Resolves #1290

Changes

Added blank and filled helpers to the helper stubs

Breaking changes

None

[canvural]

Thank you! Finally we can merge this 😄 Hope to make a release this week 👍🏽