Notifications broken if there is watched pages

If notifications are enabled, no space and no wikis are watched, but one of several pages are watched (or excluded), there is a NullPointerException.

See: https://github.com/xwiki/xwiki-platform/blob/89bf7ca9aae77cd6080989385aae6d47511dc164/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-filters/xwiki-platform-notifications-filters-api/src/main/java/org/xwiki/notifications/filters/internal/scope/ScopeNotificationFilterExpressionGenerator.java#L99

Moreover, the problem looks more serious and XWIKI-15440 should be reverted ASAP. XWIKI-15440 is also preventing a fix for XWIKI-15455.