Skip system conversations during ChatService discovery

A stored Second Life notification can appear as a direct conversation with
an all-zero participant UUID. Validate that entry but exclude it from
resident history so it no longer rejects the entire conversation list and
prevents other archives from downloading.

Cover lists containing only a system entry and mixed system/resident
entries in either order. Validated with the Release viewer build, all 116
Windows tests including 36 ChatService core cases, and GCC warning-as-error
checks of the portable core and tests.

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