fix: blob.display() shows <NA> for null rows

[tswast]

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Fixes #<issue_number_goes_here> 🦕

[shuoweil]

Please check failed testcases. Thanks a lot.

[tswast]

Test failures:

FAILED tests/system/large/blob/test_function.py::test_blob_transcribe[gemini-2.0-flash-lite-001-False]

def test_blob_transcribe(
        audio_mm_df: bpd.DataFrame,
        model_name: str,
        verbose: bool,
    ):
        actual = (
            audio_mm_df["audio"]
            .blob.audio_transcribe(
                model_name=model_name,  # type: ignore
                verbose=verbose,
            )
            .to_pandas()
        )
    
        # check relative length
        expected_text = "Now, as all books not primarily intended as picture-books consist principally of types composed to form letterpress"
        expected_len = len(expected_text)
    
        actual_text = ""
        if verbose:
            actual_text = actual[0]["content"]
        else:
            actual_text = actual[0]
>       actual_len = len(actual_text)
E       TypeError: object of type 'NAType' has no len()

It appears this test is not robust to null values. I don't think the display() method was touched by this test.

[shuoweil]

Test failures:

FAILED tests/system/large/blob/test_function.py::test_blob_transcribe[gemini-2.0-flash-lite-001-False]

def test_blob_transcribe(
        audio_mm_df: bpd.DataFrame,
        model_name: str,
        verbose: bool,
    ):
        actual = (
            audio_mm_df["audio"]
            .blob.audio_transcribe(
                model_name=model_name,  # type: ignore
                verbose=verbose,
            )
            .to_pandas()
        )
    
        # check relative length
        expected_text = "Now, as all books not primarily intended as picture-books consist principally of types composed to form letterpress"
        expected_len = len(expected_text)
    
        actual_text = ""
        if verbose:
            actual_text = actual[0]["content"]
        else:
            actual_text = actual[0]
>       actual_len = len(actual_text)
E       TypeError: object of type 'NAType' has no len()

It appears this test is not robust to null values. I don't think the display() method was touched by this test.

Test failures:

FAILED tests/system/large/blob/test_function.py::test_blob_transcribe[gemini-2.0-flash-lite-001-False]

def test_blob_transcribe(
        audio_mm_df: bpd.DataFrame,
        model_name: str,
        verbose: bool,
    ):
        actual = (
            audio_mm_df["audio"]
            .blob.audio_transcribe(
                model_name=model_name,  # type: ignore
                verbose=verbose,
            )
            .to_pandas()
        )
    
        # check relative length
        expected_text = "Now, as all books not primarily intended as picture-books consist principally of types composed to form letterpress"
        expected_len = len(expected_text)
    
        actual_text = ""
        if verbose:
            actual_text = actual[0]["content"]
        else:
            actual_text = actual[0]
>       actual_len = len(actual_text)
E       TypeError: object of type 'NAType' has no len()

It appears this test is not robust to null values. I don't think the display() method was touched by this test.

I would suggest we rebase main and try to run system tests again.

[tswast]

@shuoweil All tests are passing.