Handle Flags not present in LoanManage

- Addresses RIPD-3418

src/test/app/Loan_test.cpp

@@ -658,6 +658,12 @@ class Loan_test : public beast::unit_test::suite
         // Manage the loan
         // no-op
         env(manage(lender, keylet.key, 0));
+        {
+            // no flags
+            auto jt = manage(lender, keylet.key, 0);
+            jt.removeMember(sfFlags.getName());
+            env(jt);
+        }
         // Only the lender can manage
         env(manage(evan, keylet.key, 0), ter(tecNO_PERMISSION));
         // unknown flags

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

@@ -64,7 +64,7 @@ LoanManage::preflight(PreflightContext const& ctx)
         return temINVALID;
 
     // Flags are mutually exclusive
-    auto const flags = ctx.tx[sfFlags] & tfUniversalMask;
+    auto const flags = ctx.tx[~sfFlags].value_or(0) & tfUniversalMask;
     if ((flags & (flags - 1)) != 0)
     {
         JLOG(ctx.j.warn())