Fix some issues with the model. (google#312)

- add omitempty to more optional fields.
- correct severity, which is an array.

Signed-off-by: Caleb Brown <calebbrown@google.com>

pkg/models/vulnerability.go

@@ -54,8 +54,8 @@ type Severity struct {
 // See: https://ossf.github.io/osv-schema/#affected-fields
 type Affected struct {
 	Package           Package                `json:"package"`
-	Severity          Severity               `json:"severity,omitempty"`
-	Ranges            []Range                `json:"ranges"`
+	Severity          []Severity             `json:"severity,omitempty"`
+	Ranges            []Range                `json:"ranges,omitempty"`
 	Versions          []string               `json:"versions,omitempty"`
 	DatabaseSpecific  map[string]interface{} `json:"database_specific,omitempty"`
 	EcosystemSpecific map[string]interface{} `json:"ecosystem_specific,omitempty"`
@@ -89,11 +89,12 @@ type Vulnerability struct {
 	ID               string                 `json:"id"`
 	Modified         time.Time              `json:"modified"`
 	Published        time.Time              `json:"published"`
-	Aliases          []string               `json:"aliases"`
+	Aliases          []string               `json:"aliases,omitempty"`
 	Summary          string                 `json:"summary"`
 	Details          string                 `json:"details"`
 	Affected         []Affected             `json:"affected"`
-	References       []Reference            `json:"references"`
-	Credits          []Credit               `json:"credits"`
+	Severity         []Severity             `json:"severity,omitempty"`
+	References       []Reference            `json:"references,omitempty"`
+	Credits          []Credit               `json:"credits,omitempty"`
 	DatabaseSpecific map[string]interface{} `json:"database_specific,omitempty"`
 }