Fix identify and script scraper bugs (#2375)

* Continue identify if source fails
* Handle empty result set correctly
* Parse null values from scraper script correctly
* Omit warning when json selector value missing
* Return nil when scraped item not found
* Fix graphql validation errors

pkg/scraper/json.go

@@ -258,7 +258,10 @@ func (q *jsonQuery) runQuery(selector string) ([]string, error) {
 	value := gjson.Get(q.doc, selector)
 
 	if !value.Exists() {
-		return nil, fmt.Errorf("could not find json path '%s' in json object", selector)
+		// many possible reasons why the selector may not be in the json object
+		// and not all are errors.
+		// Just return nil
+		return nil, nil
 	}
 
 	var ret []string