Core/BattlePets: Fix nicknamed companion pets showing their species name - #32078

Summary

• The client only requests a companion's custom name when the name timestamp on the summoned unit is not 0, so a nicknamed battle pet showed its species name above its head.
• Battle pets that have a nickname but no stored timestamp now get one when they are loaded.

Test plan

• Summon a battle pet that has a nickname and confirm the nickname shows above its head instead of the species name.
• Rename a summoned battle pet and confirm the new name shows without resummoning.