Ensure that the Issues key in JSON format is a list (#2358)

This makes the data more consistent and easier to use with other tools.
For example, a third-party tool can iterate over `Issues` without having
to make a preliminary null check.

pkg/printers/json.go

@@ -30,6 +30,9 @@ func (p JSON) Print(ctx context.Context, issues []result.Issue) error {
 		Issues: issues,
 		Report: p.rd,
 	}
+	if res.Issues == nil {
+		res.Issues = []result.Issue{}
+	}
 
 	outputJSON, err := json.Marshal(res)
 	if err != nil {