refactor: Add std::optional overload for isZeroId

Addresses bthomee's review suggestion: about a third of isZeroId call
sites are the optional-field pattern 'x && isZeroId(*x)'. Add an
overload taking std::optional<T> const& and collapse the 5 direct
call sites (Payment, LoanSet, LoanBrokerCoverClawback, OfferCreate,
PermissionedDomainSet). The remaining if-let uses are left alone
because collapsing them would move unrelated code out of the outer
block.

The negated helpers (!isPositiveAmount etc.) are intentionally not
overloaded: with an optional overload, !isPositiveAmount(opt) would
be true when the optional is empty, which is the opposite of the
existing 'opt && !isPositiveAmount(*opt)' semantics.

include/xrpl/tx/helpers/PreflightHelpers.h

@@ -5,6 +5,7 @@
 #include <xrpl/protocol/UintTypes.h>
 
 #include <cstddef>
+#include <optional>
 
 namespace xrpl {
 
@@ -41,6 +42,16 @@ isZeroId(T const& id)
     return id == beast::kZero;
 }
 
+// Optional-field overload: true iff the field is present and its value is
+// unset/zero. Simplifies the `field && isZeroId(*field)` pattern into a
+// single call.
+template <class T>
+inline bool
+isZeroId(std::optional<T> const& id)
+{
+    return id.has_value() && isZeroId(*id);
+}
+
 // Checks whether an amount is a strictly positive XRP amount.
 inline bool
 isPositiveXRPAmount(STAmount const& amount)

src/libxrpl/tx/transactors/dex/OfferCreate.cpp

@@ -101,7 +101,7 @@ OfferCreate::preflight(PreflightContext const& ctx)
     // A zero DomainID is invalid for a PermissionedDomain ledger entry because
     // keylet::permissionedDomain(uint256) uses the DomainID as the ledger key.
     if (auto const domainID = tx[~sfDomainID];
-        ctx.rules.enabled(fixCleanup3_2_0) && domainID && isZeroId(*domainID))
+        ctx.rules.enabled(fixCleanup3_2_0) && isZeroId(domainID))
         return temMALFORMED;
 
     bool const bImmediateOrCancel(tx.isFlag(tfImmediateOrCancel));

src/libxrpl/tx/transactors/lending/LoanBrokerCoverClawback.cpp

@@ -49,7 +49,7 @@ LoanBrokerCoverClawback::preflight(PreflightContext const& ctx)
     if (!brokerID && !amount)
         return temINVALID;
 
-    if (brokerID && isZeroId(*brokerID))
+    if (isZeroId(brokerID))
         return temINVALID;
 
     if (amount)

src/libxrpl/tx/transactors/lending/LoanSet.cpp

@@ -147,7 +147,7 @@ LoanSet::preflight(PreflightContext const& ctx)
             return *ret;
     }
 
-    if (auto const brokerID = ctx.tx[~sfLoanBrokerID]; brokerID && isZeroId(*brokerID))
+    if (auto const brokerID = ctx.tx[~sfLoanBrokerID]; isZeroId(brokerID))
         return temINVALID;
 
     return tesSUCCESS;

src/libxrpl/tx/transactors/payment/Payment.cpp

@@ -144,7 +144,7 @@ Payment::preflight(PreflightContext const& ctx)
     // A zero DomainID is invalid for a PermissionedDomain ledger entry because
     // keylet::permissionedDomain(uint256) uses the DomainID as the ledger key.
     if (auto const domainID = tx[~sfDomainID];
-        ctx.rules.enabled(fixCleanup3_2_0) && domainID && isZeroId(*domainID))
+        ctx.rules.enabled(fixCleanup3_2_0) && isZeroId(domainID))
         return temMALFORMED;
 
     bool const partialPaymentAllowed = tx.isFlag(tfPartialPayment);

src/libxrpl/tx/transactors/permissioned_domain/PermissionedDomainSet.cpp

@@ -40,7 +40,7 @@ PermissionedDomainSet::preflight(PreflightContext const& ctx)
         return err;
 
     auto const domain = ctx.tx.at(~sfDomainID);
-    if (domain && isZeroId(*domain))
+    if (isZeroId(domain))
         return temMALFORMED;
 
     return tesSUCCESS;