Null-check pet riders in PetManager#getPet(Entity)

Previously, if any of the loaded pets didn't have a rider,
PetManager#getPet would throw a NullPointerException
Merge pull request #267 from eueln/getpetfix

Null-check pet riders in PetManager#getPet(Entity)
Fix primary key typo: Owner -> OwnerName
Merge pull request #268 from eueln/tablecreationfix

Fix primary key typo: Owner -> OwnerName