EXT-8341 FIXED Avoided assertion on receiving object auto return notification.

Details:
1 Provided passing necessary is_group parameter to LLPostponedNotification::add<LLPostponedServerObjectNotification> method call;
3 Corrected handling group name in LLPostponedNotification::onCachedNameReceived;
2 Added check on retrieved name from cache name and replacement with fallback value.

reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/743/

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branch : product-engine