Viewing graphs before the poller has run for the first time can produce unexpected errors

[TheWitness]

Describe the bug

If your settings table include an empty poller_interval, creating poller items will generate division by zero error.

To Reproduce

Steps to reproduce the behavior:

1. In mysql set the poller_interval to zero.

2. Create graphs

3. Goto the Cacti.log

4. See error

Expected behavior

Cacti should not log errors such as this.

Additional context

Cacti forum post here: https://forums.cacti.net/viewtopic.php?f=2&t=60905&p=282697#p282697