Automatically set Bulk Walk size when missing on a host

[TheWitness]

Describe the bug

This setting should allow Cacti to set the Max Repeaters at the first re-index, but due to a small coding bug, this does not happen.

To Reproduce

Steps to reproduce the behavior:

1. Set the Device to Auto-Detect at First Reindex, Save Device

2. Perform a Verbose Query

3. Note that the value does not update to an expected number.

Expected behavior

Cacti should do this automatically as expected.