do not store a change operation with no change units

src/core/Buffer.cc

@@ -353,6 +353,8 @@ void Buffer::record_change(size_t offset, size_t length, bool insert)
 void Buffer::save_current_operation()
 {
     if (m_current_change_operation)
+    {
+        if (m_current_change_operation->changes.size() > 0u)
         {
             size_t new_change_operation_index = m_change_operations.size();
             if (m_current_change_operation_index < new_change_operation_index)
@@ -360,9 +362,10 @@ void Buffer::save_current_operation()
                 m_change_operations[m_current_change_operation_index]->children.push_back(new_change_operation_index);
             }
             m_change_operations.push_back(m_current_change_operation);
-        m_current_change_operation = nullptr;
             m_current_change_operation_index = new_change_operation_index;
         }
+        m_current_change_operation = nullptr;
+    }
 }
 
 void Buffer::apply_change_operation(const ChangeOperation & change_operation, bool forward)