switches without nondefaults results in syntax error on Python
The following codes produces a syntax error on python:
```yaml
meta:
  id: ops
  endian: le
seq:      
  - id: opcode
    type: u1
  - id: arguments
    type:
      switch-on: opcode
      cases:
        _: u4
```

the produced code is:
```python
    def _read(self):
        self.opcode = self._io.read_u1()
        _on = self.opcode
        else:
            self.arguments = self._io.read_u4le()
```
hence a "else" with no "if".
Maybe this particular pattern should not be allowed ?
