Implemented avoid throwing an error if private key is missing unless .send() is executed

src/lib/account_update.ts

@@ -1877,10 +1877,11 @@ function addMissingSignatures(
         pk.equals(accountUpdate.body.publicKey).toBoolean()
       );
       if (i === -1) {
-        let pk = PublicKey.toBase58(accountUpdate.body.publicKey);
-        throw Error(
-          `addMissingSignatures: Cannot add signature for fee payer (${pk}), private key is missing.`
-        );
+        // private key is missing, but we are not throwing an error here
+        // there is a change signature will be added by the wallet
+        // if not, error will be thrown by verifyAccountUpdate
+        // while .send() execution
+        return { body, authorization: Ledger.dummySignature() }
       }
       privateKey = additionalKeys[i];
     }

src/lib/mina.ts

@@ -1257,6 +1257,14 @@ async function verifyAccountUpdate(
 
   let { commitment, fullCommitment } = transactionCommitments;
 
+  // check if addMissingSignatures failed to include a signature
+  // due to a missing private key
+  if (accountUpdate.authorization === Ledger.dummySignature()) {
+    let pk = PublicKey.toBase58(accountUpdate.body.publicKey);
+    throw Error(
+      `verifyAccountUpdate: Detected a missing signature for (${pk}), private key was missing.`
+    );
+  }
   // we are essentially only checking if the update is empty or an actual update
   function includesChange<T extends {}>(
     val: T | string | null | (string | null)[]