Concatenation with empty string is not parsed idempotently

## Steps to reproduce

Using current main (da16633812da68d2b2d8d9054030fbbda733d5a2) the following test:
```python
def test_concatenation_with_empty_string():
  # language=python
  rewrite_run(python('print("alef" "")'))
```
fails with:
```
Expected :'print("alef" "")'
Actual   :'print("alef" )'
```

AFAICT this is valid Python code.