Force storing empty bugnote statistics

(1) When a bug has no bugnotes, or visible ones, force storing a 'false'
value as bugnote stats in bug cache, to avoid querying the database again
later.

(2) Additionally, when the function bug_cache_database_result is called
with a stats array parameter, and the bug has already been cached, force
the creation of the stats index. Previously this wasn't possible if the
bug had already been cached without the bugnote stats.

Fixes 0020121