Common: fix JS enrichment failing without return statement

[chuwy]

In 1.3.0 we refactored JS enrichment to use Nashorn (#138). As a side-effect we made impossible to run JS enrichments without return statements. Although, we always recomment to use return statements and never mutate the event it's still an unwanted breaking change.

Example of enrichment failure bad row:

{
  "enrichment" : {
    "schemaKey" : "iglu:com.snowplowanalytics.snowplow/javascript_script_config/jsonschema/1-0-0",
    "identifier" : "Javascript enrichment"
  },
  "message" : {
    "error" : "Could not parse output JSON of Javascript function. Error: [null]"
  }
}