MDEV-15372: Parallel slave speedup very limited when log_slave_updates=OFF

Part #2: some transactions have m_rocksdb_tx==NULL (and most functions of
Rdb_transction_impl handle this case. Do like they do)

storage/rocksdb/ha_rocksdb.cc

@@ -2240,7 +2240,8 @@ class Rdb_transaction_impl : public Rdb_transaction {
   }
 
   void set_sync(bool sync) override {
-    m_rocksdb_tx->GetWriteOptions()->sync = sync;
+    if (m_rocksdb_tx)
+      m_rocksdb_tx->GetWriteOptions()->sync = sync;
   }
 
   void release_lock(rocksdb::ColumnFamilyHandle *const column_family,