use null_hash instead of zero variable to avoid conversions

src/math/ec_scalar.cpp

@@ -41,7 +41,7 @@ ec_scalar::ec_scalar(const ec_secret& secret)
 ec_scalar::ec_scalar(uint64_t value)
   : valid_(true)
 {
-    secret_ = zero;
+    secret_ = null_hash;
     auto serial = make_unsafe_serializer(secret_.end() - 4);
     serial.write_4_bytes_big_endian(value);
 }