fix(vulnerability): make an empty severity UNKNOWN (#759)

pkg/vulnerability/vulnerability.go

@@ -106,6 +106,11 @@ func (c Client) getVendorSeverity(vuln *types.DetectedVulnerability, source stri
 	if vs, ok := vuln.VendorSeverity[vulnerability.Nvd]; ok {
 		return vs.String(), vulnerability.Nvd
 	}
+
+	if vuln.Severity == "" {
+		return dbTypes.SeverityUnknown.String(), ""
+	}
+
 	return vuln.Severity, ""
 }