Made LLCacheName getGroupName() and getName() methods return TRUE if the given group/avatar name is NULL, so that LLNameBox doesn't display "Loading..." indefinitely.
There's nothing to wait for in case of NULL id: the name cache returns a predefined name that won't change (i.e. it's the final result).
Reviewed by Mike: https://codereview.productengine.com/secondlife/r/223/
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branch : product-engine
