Update has_content method to check content data

[dsfaccini]

FilePart.has_content() will always return True, even if it's an empty file.

@dataclass(repr=False)
class FilePart:
    """A file response from a model."""

    content: Annotated[BinaryContent, pydantic.AfterValidator(BinaryImage.narrow_type)]
    """The file content of the response."""

    ...

    def has_content(self) -> bool:
        """Return `True` if the file content is non-empty."""
        # `self.content` is a `BinaryContent` object
        return bool(self.content)  # pragma: no cover

check using

from pydantic_ai.messages import BinaryContent, FilePart

empty_filepart = FilePart(
    content=BinaryContent(data=b'', media_type='image/jpeg')
)
nonempty_filepart = FilePart(
    content=BinaryContent(data=b'abc', media_type='image/jpeg')
)

print(f'{empty_filepart.has_content()=}')        # BUG: True
print(f'{nonempty_filepart.has_content()=}')  # True