Prevent modules from returning a nil result

[phillip-stephens]

It's common when we run into an error when annotating to 1) log 2) return nil so that downstream parsers don't get a half-created result with no indication that something has gone wrong.

However, our modules were split between returning an empty result or nil, this PR adds a check in annotate.go so any nil results will be converted to an empty struct, thereby streamlining downstream processing:

Example:

echo "127.0.0.123" |  ./zannotate --censys --censys-pat="censys_xxxxXXXXXxxxxxx" --rdns --routing --routing-mrt-file=./data-snapshots/routing.mrt
{"ip":"127.0.0.123","censys":{},"rdns":{},"routing":{}}

Related Issues

Closes #78