Address FutureWarning in pandas

[cthoyt]

A similar error to #491 seems to have popped up:

/Users/cthoyt/dev/biomappings/.tox/update/lib/python3.11/site-packages/sssom/util.py:162: FutureWarning: Downcasting behavior in `replace` is deprecated and will be removed in a future version. To retain the old behavior, explicitly call `result.infer_objects(copy=False)`. To opt-in to the future behavior, set `pd.set_option('future.no_silent_downcasting', True)`
  df.replace("", np.nan, inplace=True)