Make Collector return type more explicit

[staabm]

while the previous return type was correct, it did not explicitly state/protect you from someone changing the collector and make it return a empty array.

thats a major problem which can happen, and I want to make sure this error will not bite you, like it did in my projects.

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

[staabm]

I did not apply the fix to all collector classes in this repo - but I think you get the point and can do the rest on your own.

if you have any questions left, feel free to ask.

I am not a phpstan-latte user myself, therefore I did not take care of making this thing perfect. My primary motivation was to make sure people don't shoot their own feet because of problems with Collectors