Exception when processing special character strings

When a string is passed to plural which contains only a `'s` it will raise an IndexError Exception:

```python
import inflect

p = inflect.engine()
p.plural("'s")
```

In version 5.6.0:
```
Traceback (most recent call last):
File "", line 1, in
File "/opt/_src/sqbu/voicea/wordcloud/.venv/lib/python3.11/site-packages/inflect/__init__.py", line 2376, in plural
self._pl_special_adjective(word, count)
File "/opt/_src/sqbu/voicea/wordcloud/.venv/lib/python3.11/site-packages/inflect/__init__.py", line 3102, in _pl_special_adjective
trailing_s = "" if pl[-1] == "s" else "s"
~~^^^^
IndexError: string index out of range
```

In version 6.0.4, it changes to a pydantic validation error:
```python
Traceback (most recent call last):
File "", line 1, in
File "pydantic/decorator.py", line 40, in pydantic.decorator.validate_arguments.validate.wrapper_function
File "pydantic/decorator.py", line 134, in pydantic.decorator.ValidatedFunction.call
File "pydantic/decorator.py", line 206, in pydantic.decorator.ValidatedFunction.execute
File "/opt/_src/sqbu/voicea/wordcloud/.venv/lib/python3.11/site-packages/inflect/__init__.py", line 2404, in plural
self._pl_special_adjective(word, count)
File "/opt/_src/sqbu/voicea/wordcloud/.venv/lib/python3.11/site-packages/inflect/__init__.py", line 3155, in _pl_special_adjective
pl = self.plural_noun(mo.group(1))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "pydantic/decorator.py", line 40, in pydantic.decorator.validate_arguments.validate.wrapper_function
File "pydantic/decorator.py", line 133, in pydantic.decorator.ValidatedFunction.call
File "pydantic/decorator.py", line 130, in pydantic.decorator.ValidatedFunction.init_model_instance
File "pydantic/main.py", line 341, in pydantic.main.BaseModel.__init__
pydantic.error_wrappers.ValidationError: 1 validation error for PluralNoun
text
ensure this value has at least 1 characters (type=value_error.any_str.min_length; limit_value=1)
```

https://github.com/jaraco/inflect/blob/main/inflect/__init__.py#L3153-L3157

I would expect the method to return an empty string, or otherwise catch this type of error; since technically the input is a two character string (i.e. `'s`) but the validation strips out some of that to assert.

Start at inflect/__init__.py around lines 3153-3157 and reproduce the failure with p.plural("'s"). Trace how the apostrophe-s input is extracted and validated; done means the call no longer raises an IndexError or validation error and its expected result is covered by a regression test.

Written by the indexing model from the issue text.