Handling non-data urls appropriately

Potentially return `None` in the case of an invalid data url. Would require refactoring of `__parse_url` where `DATA_URL_RE` does not match.

```
from data_url import DataURL

if url := DataURL.from_url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAA...")
    # is a data url so work with it.
else:
    pass
```

Or implement `def __bool__(self)` and return `False` when there is no match.

Thoughts?