[BUGFIX] Catch None in image feature columns (#5626)

# Description
<!-- Please include a summary of the changes and the related issue.
Please also include relevant motivation and context. List any
dependencies that are required for this change. -->

This PR fixes this snippet:

```python
import argilla as rg
from datasets import load_dataset

client = rg.Argilla(
    api_url="https://frascuchon-argilla.hf.space", api_key="argilla.apikey"
)

ds = load_dataset("derek-thomas/ScienceQA", name="default", split="train")
dataset = client.datasets(name="science_qa", workspace="argilla")

dataset.records.log(
    ds, mapping={"task": "text_0.suggestion", "subject": "text_1.suggestion"}
)
```

Closes #<issue_number>

**Type of change**
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PR according to the type of change -->

- Bug fix (non-breaking change which fixes an issue)
- New feature (non-breaking change which adds functionality)
- Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- Refactor (change restructuring the codebase without changing
functionality)
- Improvement (change adding some improvement to an existing
functionality)
- Documentation update

**How Has This Been Tested**
<!-- Please add some reference about how your feature has been tested.
-->

**Checklist**
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account -->

- I added relevant documentation
- I followed the style guidelines of this project
- I did a self-review of my code
- I made corresponding changes to the documentation
- I confirm My changes generate no new warnings
- I have added tests that prove my fix is effective or that my feature
works
- I have added relevant notes to the CHANGELOG.md file (See
https://keepachangelog.com/)

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Francisco Aranda <[email protected]>

Author: 3 people

argilla/src/argilla/_helpers/_media.py

@@ -16,18 +16,20 @@
 import io
 import warnings
 from pathlib import Path
-from typing import Union
+from typing import Union, Optional
 
 from PIL import Image
 
 
-def pil_to_data_uri(image_object: "Image") -> str:
+def pil_to_data_uri(image_object: Optional["Image"]) -> Optional[str]:
     """Convert a PIL image to a base64 data URI string.
     Parameters:
         image_object (Image): The PIL image to convert to a base64 data URI.
     Returns:
         str: The data URI string.
     """
+    if image_object is None:
+        return None
     if not isinstance(image_object, Image.Image):
         raise ValueError("The image_object must be a PIL Image object.")