`options.expirations` returns an empty DataFrame when `columns=["expirations"]` is passed

[MarketDataDev03]

Problem

Calling options.expirations with columns=["expirations"] returns a DataFrame that looks empty, even
though the data is present — it's silently moved to the index.

Reproduction

from marketdata import MarketDataClient

df = MarketDataClient(token="...").options.expirations(
    symbol="AAPL", columns=["expirations"]
)
print(df)
# Empty DataFrame
# Columns: []
# Index: [2025-12-05, 2025-12-12, ...]

Root cause

src/marketdata/resources/options/expirations.py:70 forces index_columns=["expirations"]. When the user
filters to only that column, the handler promotes the single column to the index, leaving zero data columns.

Suggested fix

Skip the set_index step when it would leave the DataFrame with zero columns (generic fix in the output
handler), or don't pass index_columns when the user explicitly filtered via columns.

Related (latent)

In INTERNAL output format, OptionsExpirations requires updated, so any partial response raises
__init__() missing 1 required positional argument: 'updated'. Worth making the field optional while in the
area.