extra warning during build with recent panda

[Remi-Gau]

Getting a lot of those.

Should fix probably soon before it becomes too annoying.

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)`
             File "/home/remi/github/bids/website/env/lib/python3.12/site-packages/pandas/core/internals/blocks.py", line 904, in replace
               warnings.warn(
             File "/home/remi/github/bids/website/env/lib/python3.12/site-packages/bidsschematools/render/utils.py", line 132, in
               df = df.replace("", math.nan).dropna(axis=1, how="all").fillna("")