Prevent panic if apk digest list is empty

psiphon/server/safetyNet.go

@@ -96,6 +96,11 @@ type jwtBody struct {
 func newJwtBody(jsonBytes []byte) (jwtBody, error) {
 	var body jwtBody
 	err := json.Unmarshal(jsonBytes, &body)
+
+	// Handle empty apk digest array
+	if len(body.ApkCertificateDigestSha256) == 0 {
+		body.ApkCertificateDigestSha256 = append(body.ApkCertificateDigestSha256, "")
+	}
 	return body, err
 }