True/false block appears after switching to Python code

Steps to reproduce: 
1. Add `if` block
2. Add `not` block and attach it to the `if`
3. Switch to Python code. After that you can see the following code

```
if not True:
  pass

```
1. After switching back to blockly interface, you can see the block `True` /  `False` attached to the `not`
