The notifications loop forever when the requested count is 0 or not a number

Steps to reproduce

Edit a page, insert the Notifications macro with a count of 0, then save and view the page.

{{notifications count="0"/}}

The same happens with a count that is not a number, like count="abc" or count="1.5".

Expected result

The macro shows no event, or an error message saying that the count must be a positive number.

Actual result

The macro stays on its loading placeholder forever, and the server keeps one CPU busy for each such page view. The asynchronous rendering job never ends: in DefaultParametrizedNotificationManager#getRawEvents, the batch size is expectedCount * 2, so 0. Each search returns an empty batch, batch.size() < batchSize is false since 0 < 0, and batchSize <<= 1 keeps it at 0, so the loop never stops, running a Solr query on every pass.

Two such stuck jobs fill the notifications job group: every Notifications macro of the wiki then stays on its placeholder until the instance is restarted, and a normal shutdown no longer completes.

Any user who can edit a page can trigger it. The REST resource /rest/notifications takes a count parameter too, and may reach the same code, which was not tried.

The loop was introduced by XWIKI-15927. Found while auditing the macro for the WYSIWYG usability of XWiki macros, on XWiki 18.8.0: https://design.xwiki.org/xwiki/bin/view/Proposal/WYSIWYGusabilityofXWikimacros/Macros/notifications/