Annotate methods that return a definitely null value with @Nullable

We intend to enable the ReturnMissingNullable errorprone pattern in
Gerrit core and this fixes the current issues.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I4bd1ff5d6cce8c1c4babe8cbd9f2266365e63987