Ignore server's last update timestamp when no programs exist in the cache

[Slugger]

A pull can be done with 0 stations requested. When this happens, future pulls to get station data will bail until the next SD server update.

This happens in SageTV when you're configuring inputs for the first time. When you assign a lineup to the input it does a pull, but with 0 stations enabled.

Need to recognize this and always do a pull when there are no programs in the cache.