Fix None type error for TI module

When user using model_name.png as a preview image, textural_inversion.py still treat it as an embeding, and didn't handle its error, just let python throw out an None type error like following:
```bash
  File "D:\Work\Dev\AI\stable-diffusion-webui\modules\textual_inversion\textual_inversion.py", line 155, in load_from_file
    name = data.get('name', name)
AttributeError: 'NoneType' object has no attribute 'get'
```

With just a simple `if data:` checking as following, there will be no error, breaks nothing, and now this module can works fine with user's preview images.
Old code:  
```python
                data = extract_image_data_embed(embed_image)
                name = data.get('name', name)
```
New code:  
```python
                data = extract_image_data_embed(embed_image)
                if data:
                    name = data.get('name', name)
                else:
                    # if data is None, means this is not an embeding, just a preview image
                    return
```

Also, since there is no more errors on textual inversion module, from now on, extra network can set "model_name.png" as preview image for embedings.