Nullness Checker unsoundly (?) assumes that method returns non-null value, despite -AuseConservativeDefaultsForUncheckedCode

A repro is included in https://github.com/VirtusLab/git-machete-intellij-plugin/pull/672/files. I can provide a more stripped-down repro if needed.

In a nutshell, I have a piece like that:

      // org.eclipse.jgit.revwalk.RevWalk walk;
      @Nullable RevCommit mergeBase = walk.next();
      LOG.debug(() -> "Detected merge base for " + c1.getHash().getHashString() + " and " + c2.getHash().getHashString()
          + " is " + mergeBase.getId().getName());

Clearly, calling mergeBase.getId() should not be allowed... yet the compilation passes without an error.

RevWalk#next is declared as:

	public RevCommit next() throws MissingObjectException,
			IncorrectObjectTypeException, IOException {
		return pending.next();
	}

I've also added -AuseConservativeDefaultsForUncheckedCode=+bytecode, to no avail :/

Start with the linked git-machete-intellij-plugin pull-request reproducer and the Nullness Checker treatment of org.eclipse.jgit.revwalk.RevWalk#next. Check how -AuseConservativeDefaultsForUncheckedCode=+bytecode affects the inferred return nullness. Done means the reproducer no longer compiles without a nullness error, with a regression test if the project provides a suitable location.