Align modules on error behavior

[phillip-stephens]

Currently if an error is encountered (or even if the data source just doesn't have any info for a given IP), some modules return nil and some return an empty ModuleResult struct.

IMO, we should coalesce around a single approach that we define in the README, and returning nil (parsed into null by the json encoder) seems ideal.

[zakird]

IIRC, it's bad practice to return NULL in JSON instead of empty record {} or leaving out all together, as it breaks downstream processing.