Add default constructor in CertificateRequest

When fetch CertificateRequest instance from storage, we may want a default constructor to create an empty instance.

Change-Id: I3be7517465fc912813282d7b40f9de3d9422cb70
diff --git a/src/certificate-request.cpp b/src/certificate-request.cpp
index d1a4545..ac1a216 100644
--- a/src/certificate-request.cpp
+++ b/src/certificate-request.cpp
@@ -24,6 +24,8 @@
 namespace ndn {
 namespace ndncert {
 
+CertificateRequest::CertificateRequest() = default;
+
 CertificateRequest::CertificateRequest(const Name& caName,
                                        const std::string& requestId,
                                        const security::v2::Certificate& cert)