Concatenation with empty string is not parsed idempotently

[greg-at-moderne]

Steps to reproduce

Using current main (da16633) the following test:

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.