Nullity annotations on .api

[xenoterracide]

Description

I notice that jgit's use of NonNullApi support is incomplete... specifically .api is not annotated and methods may return null.

This request is to annotate the api package.

Additionally it would be nice if you switched to or additionally included @Nullmarked either from jspecify or adding that annotation yourself

Motivation

Not having this prevents tools like errorprone with nullaway from working correctly. Also, adding Nullmarked is supported by nullaway for autodetection, where at the time of this writing NonNullApi is not uber/NullAway#1084 . In lieu of using Nullmarked I have to look at the source and then manually include/exclude subpackages that are marked.

Alternatives considered

No response

Additional context

I'd be happy to do a partial PR for the methods I know about, and add the NullMarked or include jspecify.