writing text does not correctly handle zero length strings

The following creates incorrect output:
```python
import roffio

contents = {"tagname": [("keyname", "")]}

roffio.write("file.roff", contents, roff_format=roffio.Format.ASCII)
print(roffio.read("file.roff"))
```
produces `{'filedata': {'byteswaptest': 1, 'creationDate': '13/12/2024 11:54:11'}, 'version': {'major': 2, 'minor': 0}, 'tagname': {'keyname': '"'}, 'eof': {}}`

Start by running the Python example and inspecting the roffio.write/read round trip for the empty string in the tagname/keyname entry. Trace how zero-length text is written and read; done means the round trip preserves the empty string instead of producing a quote character.

Written by the indexing model from the issue text.