When restoring from backup, RRDTool may throw errors until the next poller run

[TheWitness]

Describe the bug

When you take a backup file from an old system and attempt to restore it to a new system, RRDtool may crash for a time until all pollers have been run successfully at least once. This only happens for tools like gexport that do not set their own starting and ending graph timestamps.

To Reproduce

Steps to reproduce the behavior:

1. Take a backup from a system that has not seen an update in a few days

2. Restore it as like a disaster recovery restore

3. Before starting the data collectors, attempt to view some graphs

4. Note in the Cacti log that there will be errors about RRDtool crashes

Expected behavior

Cacti should compensate for this timestamp issue.