SERVER-60752 made constructInstance take an 'existingInstances' parameter so that services can check if the new instance would conflict with any existing instances.

However, constructInstance is called in both PrimaryOnlyService::getOrCreateInstance and PrimaryOnlyService::_rebuildInstances. There's no need to check for conflicting instances in _rebuildInstances, so we pass an empty 'existingInstances' vector.

Instead, we should make a separate method 'checkIfConflictsWithOtherInstances' that is only called on getOrCreateInstance.