fix issue where if there are not two notarisations the MoMoM is indeterninate

src/crosschain.cpp

@@ -81,6 +81,11 @@ uint256 CalculateProofRoot(const char* symbol, uint32_t targetCCid, int kmdHeigh
         }
     }
     
+    // Not enough own notarisations found to return determinate MoMoM
+    destNotarisationTxid = uint256();
+    moms.clear();
+    return uint256();
+
 end:
     return GetMerkleRoot(moms);
 }