Massive decrease in poller performance due to unset variable

[TheWitness]

Describe the bug

Cacti Fix #3808 introduced a regression in the read_config_option() function call that would result in a massive decrease in poller performance in cases where the variable was not set in the database. This increases both query load, and decreases poller performance.

To Reproduce

Steps to reproduce the behavior:

1. Note your data collector timing.
2. unset the 'selective_debug' variable in the settings table via 'delete from settings where name = "selective_debug"'
3. Note your new data collector timing

Expected behavior

Poller performance should stay nimble when variables are not set.