fix: Check for empty or zero VaultID in LoanBrokerSet

- Resolves RIPD-4067.

src/test/app/LoanBroker_test.cpp

@@ -915,6 +915,17 @@ class LoanBroker_test : public beast::unit_test::suite
             // test
             env(jv, txflags(tfFullyCanonicalSig), ter(temINVALID));
         };
+        auto testZeroVaultID = [&](auto&& getTxJv) {
+            auto jv = getTxJv();
+            // empty broker ID
+            jv[sfVaultID] = "";
+            env(jv, ter(temINVALID));
+            // zero broker ID
+            jv[sfVaultID] = to_string(uint256{});
+            // needs a flag to distinguish the parsed STTx from the prior
+            // test
+            env(jv, txflags(tfFullyCanonicalSig), ter(temINVALID));
+        };
 
         if (brokerTest == CoverDeposit)
         {
@@ -1058,6 +1069,12 @@ class LoanBroker_test : public beast::unit_test::suite
                     set(alice, vaultInfo.vaultID),
                     loanBrokerID(brokerKeylet.key));
             });
+            // preflight: temINVALID (empty/zero vault id)
+            testZeroVaultID([&]() {
+                return env.json(
+                    set(alice, vaultInfo.vaultID),
+                    loanBrokerID(brokerKeylet.key));
+            });
 
             if (asset.holds<Issue>())
             {

src/xrpld/app/tx/detail/LoanBrokerSet.cpp

@@ -40,6 +40,12 @@ LoanBrokerSet::preflight(PreflightContext const& ctx)
             return temINVALID;
     }
 
+    if (auto const vaultID = tx.at(~sfVaultID))
+    {
+        if (*vaultID == beast::zero)
+            return temINVALID;
+    }
+
     {
         auto const minimumZero = tx[~sfCoverRateMinimum].value_or(0) == 0;
         auto const liquidationZero =