Bug: empty string generated for regex requiring at least one char

### Description

Regex seems to be ignored inside dicts, as Polyfactory is generating `` as a value:

{'environment_variables': {'XZdrHqToIHlKKDxepfKJ': ''}}

Calling `build()` 10 times results in `None` 6 times, and `` 4 times.

--

This does NOT happen when the value is not nullable, i.e.:

```
class Model(BaseModel):
environment_variables: Dict[str, constr(regex=r"^[A-Za-z_]+$")]
```

--

This ALSO happens when key/value are swapped:

```
class Model(BaseModel):
environment_variables: Optional[Dict[constr(regex=r"^[A-Za-z_]+$"), str]]
```

Result:

{'environment_variables': {'': 'BildVEhMalDZKoEqvGvl'}}

--

This does NOT happen outside a dict:

```
class Model(BaseModel):
environment_variables: constr(regex=r"^[A-Za-z_]+$")
```

Result:

{'environment_variables': 'XCjdsB'}

### URL to code causing the issue

_No response_

### MCVE

```python
from polyfactory.factories.pydantic_factory import ModelFactory
from pydantic import BaseModel
from typing import Optional,Dict
from pydantic import constr,Field

class Model(BaseModel):
environment_variables: Optional[Dict[str, constr(regex=r"^[A-Za-z_]+$")]]

class Factory(ModelFactory[Model]):
...

print(Factory.build())
```

### Steps to reproduce

```bash
See MRE
```

### Screenshots

_No response_

### Logs

```bash

```

### Release Version

2.21.0

### Platform

- [ ] Linux
- [x] Mac
- [ ] Windows
- [ ] Other (Please specify in the description above)

Start with the MCVE using ModelFactory[Model] and the Pydantic model shown in the issue, then inspect how dictionary keys and values are generated for Optional fields. Reproduce the output across repeated builds; done means regex-constrained dictionary keys and values never become empty strings.

Written by the indexing model from the issue text.