Add nullability annotation based on JavaDoc

[schnapster]

This morning I was greeted by a nice error in our production service:

java.lang.NullPointerException: getCookies(...) must not be null

Oof.

The docs of HttpServletRequest#getCookies() clearly state it may return null, however its 2024 and

• noone reads docs, speaking strictly for myself, anyways =)
• since early 2022, Jakarta Annotations v2.1 was released, and along with it nullability annotations are officially available in the Jakarta ecosystem

I have therefore given the JavaDocs of Jakarta Servlet a cursory look and implemented nullability annotations where I deemed them sufficiently indicated.

This should help all devs using modern IDEs and modern compilers (e.g. Kotlin): they will see warnings when accessing nullable values without explicit null checks.

I've probably missed some places, but I hope this PR by itself already provides enough value to the project to be merged.

Please let me know with any feedback or anything missing.