Add optional bool to disable computation of proof in JSDescription constructor

src/zcash/JoinSplit.cpp

@@ -173,9 +173,10 @@ public:
         boost::array<uint256, NumOutputs>& out_commitments,
         uint64_t vpub_old,
         uint64_t vpub_new,
-        const uint256& rt
+        const uint256& rt,
+        bool computeProof
     ) {
-        if (!pk) {
+        if (computeProof && !pk) {
             throw std::runtime_error("JoinSplit proving key not loaded");
         }
 
@@ -231,6 +232,10 @@ public:
             out_macs[i] = PRF_pk(inputs[i].key, i, h_sig);
         }
 
+        if (!computeProof) {
+            return ZCProof();
+        }
+
         protoboard<FieldT> pb;
         {
             joinsplit_gadget<FieldT, NumInputs, NumOutputs> g(pb);