[Transaction.cpp] small modification to xaccTransSetReadOnly

if reason is "" then remove the kvp

libgnucash/engine/Transaction.cpp

@@ -2015,16 +2015,7 @@ void
 xaccTransSetReadOnly (Transaction *trans, const char *reason)
 {
     if (trans && reason)
-    {
-        GValue v = G_VALUE_INIT;
-        g_value_init (&v, G_TYPE_STRING);
-        g_value_set_static_string (&v, reason);
-        xaccTransBeginEdit(trans);
-        qof_instance_set_kvp (QOF_INSTANCE (trans), &v, 1, TRANS_READ_ONLY_REASON);
-        qof_instance_set_dirty(QOF_INSTANCE(trans));
-        g_value_unset (&v);
-        xaccTransCommitEdit(trans);
-    }
+        set_kvp_string_path (trans, {TRANS_READ_ONLY_REASON}, reason);
 }
 
 /********************************************************************\

libgnucash/engine/test/utest-Transaction.cpp

@@ -1764,8 +1764,8 @@ test_xaccTransGetReadOnly (Fixture *fixture, gconstpointer pData)
     g_assert_null (frame->get_slot({TRANS_READ_ONLY_REASON}));
 
     xaccTransSetReadOnly (txn, "");
-    g_assert_cmpstr (xaccTransGetReadOnly (txn), ==, "");
-    g_assert_cmpstr (frame->get_slot({TRANS_READ_ONLY_REASON})->get<const char*>(), ==, "");
+    g_assert_cmpstr (xaccTransGetReadOnly (txn), ==, nullptr);
+    g_assert_null (frame->get_slot({TRANS_READ_ONLY_REASON}));
 
     xaccTransClearReadOnly (txn);
     g_assert_null (frame->get_slot({TRANS_READ_ONLY_REASON}));